I've planned a few of these

The Bachelorette Party: We rode around town on our MTBs. Every attendee had a veil, white gloves, and flowers on her handlebars. Followed by champagne and snacks at a friend's house. We called it "The Bride Ride".

The Wedding: I attended a wedding at the JP's where each guest (and the bride and groom) rode to the wedding.

Where to register?: At your LBS of course. Pick out the tandem of your dreams!

Start/End of a Ride/Race/Event: Did you meet your sweetie at the local Century? A yearly MTB race? Plan to wed there.

Cake Topper: I'm sure a "Google" will turn up a link; if not consider fabricating your own. We did a groom's cake (chocolate w/ chocolate frosting) that had MTBing Teddy Grahms all over it one time. Cute and tasty!

Casual Catering: ( perhaps the rehersal dinner?)....bike-shaped pasta and Fat-Tire Ale of course!

Have fuuuuun!

(and congratualtions!)