The Manifold Valley in Derbyshire has an 8.5 mile cycle trail that follows the line of an old railway line through the valley. It's a linear ride, so you will end up cycling the trail both ways, but it's so enjoyable that this isn't a disadvantage.
There is a short section along a quiet lane and the ride is almost completely flat, so it's suitable for children. you can begin either at Hulme End or Waterhouses and there is ample car parking at either end. My personal preference is to start at Hulme End because the ride to Waterhouses is very slightly uphill, so it's downhill almost all the way back!
There is a cafe at Hulme End, which is closed on Mondays, one at Wetton Mill and another one at a farm about a mile from the Waterhouses end of the trail.
The trail passes close to Beeston Tor which has some classic rocks climbs. There is also Thor's Cave up on the hillside. You can park the bikes at the bottom and follow an obvious path that climbs up to the cave, where you get an excellent view over the Manifold Valley. The cave was probably occupied by humans as long as 10,000 years ago. Stone tools and the remains of long extinct animals have been found inside the cave.
The trail is very popular with walkers and cyclists. However, if you get up early you can have it almost completely to yourself.
There are several camp sites near the Hulme End of the trail, including one in a field behind the pub. We have stayed at the Wallbrook House site a couple of times. the facilities are basic, but very clean and the owners (Mr & Mrs Goodwin) very friendly. The site is a single field, which both times we have stayed there has been very quiet. The village shop and the pub in Hulme End are about a mile away. One of the best features of the site from our point of view is that's really in the middle of nowhere.
The George at Alstonfield is only about three miles away from Hulme End. It's an old family run pub that serves locally sourced food. The interior has been restored with lime plaster and old furniture.