One of my favouritest day-trips in all the world is a reservoir circuit in the Peak District. Three hours takes you round a chain of reservoirs (Ladybower, Derwent, Howden) just north of the Hope Valley. It's day-trippable from London (to Bamford, change at Sheffield: 3hr each way; I did my return the other day, with bike, for £25). There's a map below.
The scenery is awesome, the dam engineering impressive, and the route is essentially flat. It's a mixture of tarmac (roads which are traffic-free at weekends, and quiet at other times) and gravel paths, best on a mountain bike, but doable on a hybrid or sturdy tourer. There's a cafe a third of the way up where you can refuel on pasties, and a crossing-point between sides.
Stop off at the Yorkshire Bridge pub on the way back to the station and enjoy a pint of Pale Rider; even if you've had Peaks drizzle all day, you won't be a pale rider for long.
Here's the Google map. You can go either way round; the right-hand side is mostly track, the left-hand side all tarmac, and it's gently downhill from top to bottom.
If you're on a road bike, do it anticlockwise to finish off with some nice smooth downhill; on a mountain bike, do it clockwise to enjoy a more crunchy final descent. There are a few slopes but not really any hills.
For a treat after your ride, take a dip in the remarkable outdoor heated pool at Hathersage (open summer only) and take the train back from there. You can swim surrounded by lovely Hope Valley peaks in this charming village, which has an encouraging 2:1 ratio of outdoor shops to estate agents.
There's a curiosity nearby: the collapsed road of the old A625 round the side of Mam Tor, which looks like an earthquake zone in some remote Lonely Planet guide. (It's the other route on the map.) This was one of the chapters in my book.
View larger map