Myakka is a great spot - I've been there many times. The last time I was there, the water was getting really low and the place was FULL of people so I'm not surprised that it was that low when you were there. It is a bit shocking to see just the same. I remember when it flooded out a few years ago and wiped out the boardwalk (bird walk).
I was thinking of hitting Highlands Hammock State Park (outside of Seabring), the Econ River Wilderness area, and perhaps a few other places in this area. I'll report back on Tuesday on what I found. Highlands has lots of remote places so it's not overly crowded typicaly and there are a variety of places to visit. The Econ has some people, mostly mountain bikers, but they are typicaly spread out and mostly unobtrusive, plus there is the old railroad bridge (Flagler's) as well as some great river opportunities.