An idea that I’ve been toying with in mid Wales (not sure if it would work elsewhere) is to have a ride with the aim of crossing as many different rivers (or tributaries thereof) as possible.
So you could start at Mach and cross one of the Dyfi tributaries on the way to the Severn round Havren way, then cross the Upper Wye and over to Cwm Ystwyth just touching the little streams that drain off Cefn Groes into the Rheidol, before heading towards the Teifi and Tywi. If you went East to the Elan that wouldn’t count again, as it is a tributary of the Wye. See what I mean?
It would be open to different strategies as the shortest route would involve clambering over bogs and steep hills near the sources with a bike on your shoulder, or a much longer but easier ride (on a road bike even),helping to negate the concept of a “winner”.
This is what I spend my time at work thinking about, anyway.