I made a couple T-shirts for Ben for our cotton (second) anniversary.
Once I realized two years was cotton I immediately had a ton of ideas. In the end I settled on two shirts. They both have a book/library theme which worked well since we had book and library elements in our wedding and because Ben is now a working librarian.
I made one shirt with a library card pocket with the book being checkout out on our wedding and anniversary dates.
I used Cafe Press to print the shirts and I was very happy with how they turned out.