Many individuals just respond with "Thank you." or "Thanks." "It's lovely to see you, too," I'd say occasionally. (If I am truly delighted to meet that individual again, I'll probably also throw in a comment about how great it is to see them again.) Sometimes I will add a special word or phrase if the person who greeted me has been particularly kind or helpful during my stay. For example, if a hotel clerk has gone out of her way to make my stay pleasant, I might say thank you very much and give her a small tip. If my host takes me out to dinner or a movie, I'll usually get something nice in return - perhaps a phone call from him or her later on! There are many other ways to say thank you, such as writing a letter or email, giving someone a gift, doing them a favor, etc. The key is to find something that feels right for you and your relationship with this person. Have fun saying thank you and don't forget about reciprocity. It's a beautiful thing!