@fBoyle-- Does your co-worker like to read? I think that a book is a great, inexpensive gift idea for a co-worker. If you're not sure about what type of books they enjoy reading, you can purchase a gift card for a bookstore.
I always appreciate gift cards. I like picking out my own gifts and you can never go wrong with books. One can almost always find something they like from a bookstore. Or you could get a gift card for a coffee shop. Nice and different coffee mugs make great gifts too.