How to say thank you

My boyfriend's family invited me along on a family trip for a four day vacation. Whenever we do anything, they don't allow me to pay for meals,offer to buy things like tshirts, and pay for gas. Being in college and living on a tight budget, I really appreciate it. However, I don't always know how to show just how grateful I am. I've began bringing homemade dishes as thank yous when we hang out, which they appreciate. The family vacation will again be little cost to me. What can I do as an appropriate thank you for something big, like the vacation?