Around the Avon Dam, maybe? There’s a gentle tarmaced traffic free road up towards it and some gentle (in most places) trails around the water itself.
Or, if you’re feeling more adventurous, the Old Puffing Billy tram line up to Redlake. Eight miles there and fairly gentle, except at the start where it’s a bit steeper, with a cracking ride back down the next morning & plenty of great views & camping options at the top.
Redlake
Get googling for more info.
PS: You can have a dip in the streams above the Avon Dam but I’m not sure about the ‘pond’ at Redlake. It looks a bit like it might contain contaminants from its mining heritage. Also, the area around the Avon Dam is quite popular at times so it won’t exactly be wild during the hours of daylight but it’s a great camping spot and easily accessible 🙂