I've thought about RAGBRAI: classic event aimed at bike-camping, cost is pretty low, challenging course etc. But one has to register months ahead & hope to win a lottery? My favorite mass ride was back when George Washington Parkway, between Alexandria VA & Mt Vernon, closed off one side of the road on Sundays for cyclists. There was already a parallel bike path but it was narrow crushed limestone. On the Sunday rides thousands of bikers rode: no fee, no special rules. ~20 mile round trip so casual bikers & families enjoyed a bit of challenge but with smooth safe roads. Even shown in Nat Geographic magazine. Also one side of the Chesapeake Bay Bridge used to be closed off one day for bikes/pedestrians, hugely popular but I guess tourism industry didn't like it.