Well, Hamburg is clearly the place to be this weekend: there's the 34th Alstervergnügen, whatever that is, but it looks fun, and features cool aqua-bikes (scroll down to the Wasser-Fahrrad section).
(The bikes might come in handy for the residents of Greenwich, whose tunnel - used by hundreds of cyclists per day - is now being closed off for long periods of refurbishment, with no alternative to get a bike across the Thames apart from several miles' detour to Woolwich or Rotherhithe. They could just ride across the river instead)
In England, the home of bog snorkelling, we seem to prefer underwater biking, as demonstrated by a woman in Brighton in June. Sorry, but I'm with the Germans on this one. Biking on top of water, and enjoying a few sausages and beers as you wait patiently for the main verb: sounds a fine way to spend a bank holiday weekend to me.