With two days, you'll be able to ride both the south loop and north loops of the Kalkaska trail. The south loop is shorter and probably a good one to start on.
The Frederic loop is not bad if you park on the newer lot on Couny 612. The loop that crosses 612 used to be really nice tight stuff, and still may be, but I haven't been on it for a couple years. The bad thing about Frederic is that it's close to horse trail, and often people ride their horses on this trail and really rip it up.