Thank you gifts? Help!

My parents are watching my two girls (almost 2yo and 8 month old) for 4 nights while my husband and I attend a trip that his company gifted us.

My girls are easy in every sense of the word but it's still a LOT of work to just entertain and clean up after them. We will have all meals and milk set up. My parents are staying here to watch them.

What sort of gifts or thank yous should we give? They won't accept money but I want to do something nice to let them know how much we appreciate them.