In reply to Nicholas Livesey:
Yep - Moelwyns West of Blaenau Ffestiniog / North of Cnicht is a good place - lots of minor bumps, small llyns and streams flowing in seemingly random directions (and also lots of confusing <5m features that are obvious on the ground but not on the map).
Parts of the Migneint and Rhinog areas would be similar.
Carneddau are good but on a much larger scale - you have to make a pretty major error to end up down the wrong river valley!
The Gwydir forest would be another possibility - forestry tracks with limited visibility of the surrounding terrain so no nice summits for taking bearings.