At the back of my work is a wee offroad loop I sometimes play on, it has a few short steep climbs and a couple of interesting little techy decents. On a geared XC type race bike going full speed it takes about 15 minutes per lap but it's not really very difficult. On my rigid singlespeed it takes a few more minutes but is more challenging on the climbs and decents. If I make the SS into a fixie it becomes even more difficult, and if I try not to use the brakes I really struggle in places... but if it's dry and I am having a good day I can still ride all of it.
Riding SS or fixie is a way of increasing the challenge without having to drive further, make new trails, or push the speed up.
It's all fun but the smiles per miles are higher for me when I'm using more of the skills I've built up over the years. I like the challenge.