My sister and her boyfriend recently went to a wedding. It was his friend and yet my sister got stuck picking the gift. He wanted to get something "different" and not on the registry
There's a reason people register--so they don't get "fun" or "different" things. I went shopping with my sister and we got three items off the registry and Linens n Things. DONE.
Since this is your brother, you can go with anything. A shadow box using their invite and pics of them together would be fine.