I haven't been to Cassoday in a couple years, but it used to be the first Sunday of every month during riding season. I heard a rumor last year that the town was trying to shut it down, due to a minority of folks who couldn't control their bikes or bladder. There would normally be 1-2K bikes there on a given month!
Here is a nice route on 4 highway west of Topeka, to 99 up to Alma. Quite scenic.
KS-4 E to E 7th St - Google Maps