Long ago I learned that if you loaned someone a pen that needed a cap, and kept the cap, they were unlikely to accidentally put the pen in their pocket when they were finished using it.
This is one situation where I would consider getting pressurized aerosol containers, which guests will not be able to get on the airplane for the return flight.
I really like sapphiresteve's response. Our villa is in Curacao, so sunscreen and repellent is a must We have a basket outside under the covered lanai that contains all sorts of these bottles left by other guests. Leave the cap off of these products and they will most likely be returned into the basket upon departure. It also encourages others to leave their supply, leaving room for more bought goodies.
While leaving the cap off a bottle of suntan lotion may discourage guests from taking the bottle with them, to me it looks a bit tacky to see a group of uncapped bottles sitting around. You could have sand or hair stick to the uncapped openings and that would be really a turn-off to some guests if the housekeeper doesn't make sure to keep the cap areas clean.
I'd suggest putting together an attractive basket for sunscreen and attach a laminated tag that encourages them to help themselves and please put the lotion back in the basket for the next guests to use. You could also add a line that says if they would rather not travel back home with sunscreen that they've purchased, they're most welcome to leave it in the basket for others. An attractive, professional presentation will go a long way to encourage guests to be considerate
I provide a supply of basic necessities - napkins, paper towels, toilet paper, basic cleaning supplies, kleenex, etc. In my owners book, I outline the items they should find at check in, and then I state that when they depart from the home, they must make sure they have left the same supply/amount of these items so they are available for the next guest.
Also, if you have it outlined what should be in the house upon their arrival, they will know a former guest did not re-supply or left with the item - you as a homeowner were conscientious and originally supplied the items.
On the whole, it works fairly well. When I am at the house, I always make sure there is an adequate supply. The only item there really seems to be issues with is toilet paper. Originally I bought a large supply of it. I don't think anyone ever replaced it because I had supplied so much. Because it is extremely expensive, now I go to the dollar store and leave enough to get by...
I did this, because when we rented homes in the past, in most cases we got there, and there were NO supplies, and the first thing we had to do after traveling all day was to go out and find supplies.
We leave a supply of toilet paper, paper towels, dish soap, hand soap, Cascade, laundry detergent, and fabric softener, etc. for each guest, each time, and don't expect to have the guest replenish before they depart. I try to keep in mind how I would feel as a guest to have to go shopping and replenish items before I leave for home.
I also keep in mind how put out I felt when I visited a condo in Hilton Head that provided 1 dishwasher packet and a small bottle of dish soap, and NO HAND SOAP for an entire week's stay. I had to buy a box of dishwasher soap as well as hand soap for each bathroom so we could wash our hands and dishes!
When we visit our own home, I try to make sure there is also plastic wrap and tin foil, as well as sandwich baggies in supply, but don't advertise that or ask guests to replenish what they use. A lot of guests will leave their leftover supplies, as well as leftover sugar, flour, creamer, coffee, syrup, and more.
I consider this to be a bonus amenity and ones that guests would not normally expect. My choice would be to leave it out as long as guests can conveniently purchase their own sunscreen nearby. One of the reasons is due to the typically high cost of sunscreens and sunblocks. As it is proving to be an increasing cost to you it appears as though many guests are choosing to just take it with them.
Provide as many amenities as you can, especially those that do not need to be "replenished", like hair dryers, electronics, books, games, accessories in the kitchen, etc. When it comes to replenishable amenities we include soap, shampoo, conditioner, laundry soap, dryer sheets, dishwasher detergent, dish soap, dish sponge, paper towels, etc.
I used to offer a basket ( in both baths) filled with small bottles of shampoo, toothbrushes, sunscreen, first aid supplies, etc. It got pretty expensive when I realized many guests just thought these were freebies for them to take home.
SO, I added a small sign on each basket " Please, use only what you need. There is a list of stores close by in the Wecolme Book on the hall table."
So far, it seems to have slowed things down a bit. Who goes to the beach without sunscreen, or a toothbrush? You can also find "one time use" toothbrushes at most discount stores. I know some folks need emergency supplies, but I'm not subsidizing "personal products" any more than I'd offer bathing suits.
I do keep liquid bath and hand soap in each bath ( and have my housekeeper refill them from a large bottle I get at Costco ).
The kitchen is stocked with basic items like foil, baggies, etc. Dollar sotres are great places to purchase these.
We advertise that we provide starter supplies of paper towels, TP and liquid hand soap - that's it. My word, where does it end? I also have a little note to our guests, something along the lines of "....if you use it, put it back or replace it, as the case may be..." We ususally do an early spring trip to get our home ready for the busier seasons, and I may leave some bug spray and sun screen where it's accessible. However, I'm not going to supply every possible amenity that a person could desire. I think guests have some responsibility too, and it may be that they have to replenish the garbage bags if they're the person using up the last one.
Our VR is in the Caribbean. We do not supply sunscreen, bug spray or other personal products, but if a previous guest leaves some (and most do), it stays for the next guest. We do supply toilet paper, paper towels, kleenex and napkins, Laundry soap, dish liquid, dishwasher detergent, hand soap, bath soap in reasonable quantities. If the guest runs out before the weekly housekeeping visit, they need to buy their own. Most do this and do not complain. If they insist we supply more, they can call up housekeeping and we don't quibble. Because they must arrive and return via airplane, we find they leave more than they take except for small shampoos, conditioners, body lotions which we used to have as a gift. We don't have those anymore as it just got too expensive and hard to keep track of quantities.
I would not supply these things. There are so many different types, SPFs, etc.
This is not something guests expect and most probably prefer to have their own anyway.
If anything, i would put a friendly reminder in any welcome info that is sent to "not forget sunscreen and bug spray"
I'm guessing this is not something that you advertise to promote bookings and its not something guests expect so it brings you limited if any benefit and may create a negative impression if it runs out, bottles get greasy, etc.
Regarding everyone who chimed in on dish soap/paper products, etc.
My persoinal feeling is that these things should be case dependent.
If most guests are flying to get to the property, I would probably look to supply consumables.
In my case, I have a 4 BR house at the beach that is 2+ hours from a major airport and every single guest I've rented to in 5 years has driven to get there. The house sleeps 10-12 and I only do weekly rentals in the summer. I ask all guests to leave partial rolls for the convenience of the next guests at arrival, but note in my rental info that "consumables" (soap, paper products) are NOT provided and to either bring them or plan to get to to the on-site grocery store soon after arrival.
This is not something I advertise on my page, but rather make sure guests know by sending it in welcome info before they arrive. I have never had anyone complain or even mention it. I have had many repeat guests as well so I don't think it's detrimental.