I love wedding planning! I found a couple of websites for custom-made cake toppers with bikes: http://www.lymans.com/bike/index2.htm (click on "caketops"), and http://pikespeakmall.com/caketoppers/

I've also seen lots of sites with ceramic and crystal figurines. You can look for light, appropriately sized bicycle figurine and use that as your cake topper (try a google search). I like the toys idea too. My friend works at Mattel and he had his pals there make ken/barbie dolls that looked like him and his (then) fiance, which they used as their cake topper.

If you haven't already, I recommend you also go to the message boards on wedding sites like http://www.theknot.com. You can get a lot helpful tips and info/resources from the women there.

Again, congratulations!