Are you drinkers? I think wine or whiskey mixing ceremonies are fun and unique. I'm also a fan of tree planting ceremonies. If you don't find something that appeals to you as a couple, I would honestly just skip it. In my opinion, vowing to spend the rest of your life with someone is uniting enough.
It's very similar to a sand ceremony. We'll each have two different colors of glass, but after the wedding we'll ship the container back to the company and they'll make it into a hand-blown glass art piece.
One I have seen is putting together a puzzle, but it is for blended families or families with children. You could probably do pets names if you have any pets. Another one I have seen is a painting ceremony. Basically one person as one color of paint and the other person has another and you pour the paint on the canvas.
We are doing a time capsule.. similar to the wine/letter box only without the wine and including photos and letters from our families as well. FH is amazing at wood working, so the box will be a piece he made from scratch.