Just as the title says really. Can be serious or frivolous!
For me: I absolutely adored the dolls house I got as a kid one Christmas. I still think of it decades later. And a surprise package of sweeties, beer and a book when I was feeling sad.
One of the best I've given was a bauble for a Christmas tree. Personalised with the name of a late sister and with a charity badge inside that was relevant to her work.