I can recommend the Wellow Woods Nature Trail (GC1XTYN) series of caches and a few others nearby in the woods near Ollerton in Nottinghamshire. Some really interesting containers & locations!
Jacaru & Northking's cache's are always well thought out.
Jacaru's ones at Colwick Park are enjoyable, Colwick woods is too, but more difficult.
There's a series in Gonalston (Romp around Gonalston GC1MHJP) which is a nice circular walk, and I did one just yesterday around Hoveringham (Hike around Hoveringham GC1QNVE), part of which is alongside the trent with I really enjoyed.
And if you want to try and find a really tricky one, try Going for Ground (GC20713), which although not in the countryside will have you hunting for a while (well it certainly did for us!)
Ben
Mr. Rabble