I LOVE the idea and decided I was gonna do it when I first signed up for Rover. Not only is a great perk for pet-parents who are deciding on a sitter, but it's a way to show your appreciation that the pet-parents trusted you with their baby! Here are some tips that I've found helpful when putting a goodie bag together:
As you mentioned, treats all depend on the dog's diet and allergies, etc. Ask the pet-parents what their dog usually eats for treats, and if it's not too expensive, buy some to throw in the bag.
A printed picture of the dog's stay is a wonderful idea! Not gonna lie, I didn't think of it until you said it, but I will definitely steal that idea!
Keep up the rapport with some good marketing - include a little note saying thank you, perhaps with a little description of how great their dog was. Then end it with a friendly reminder to post a review for you if they were satisfied with their dog's stay.
Of course, try to keep the cost low! If you can, buy little toys/treats in bulk so that you have your "generic" goodie bag, but then you can modify it slightly depending on the dog's specific preferences and needs.
I'm still new to Rover, so it's a method that needs some real-life testing so to speak, but I think it's a good way to set you apart from other sitters and may even increase your chances of getting more good reviews and repeat customers! Good luck!
I try to do different things, depending on the client. I have given stickers to alert Fire rescue, Guides to what fruits/veggies dogs like, printed extra-good pictures, poop-bag holders, or any small useful items I can think of.