Alafia and Boyette are great trail systems. You could try Rollercoaster, and head out at the "bailout" at the first clearing. That pretty much shortens that trail quite a bit.
The first half of Bridges isn't too bad either. The tough climb is on the second half.
As mentioned earlier, sand pine, rock garden, and River loop. Just be careful on rock garden and river loop after the rain. There are some spots that can stay wet for days, and riding through those areas causes damage.
There is also a short blue off of rock garden, garden ridge I believe, that is a good learning trail. Just be careful, it is a two way trail.