I live about an hour or so south of Dayton (Northern Kentucky). I've never fished the Mad but have fished the Little Miami in a few different places. It's a pretty typical midwest stream. Smallmouth, Largemouth Bass, Pumpkinseed, bluegill, rockbass, catfish, some carp, some white bass and in some areas, hybrids, etc. Nice stream. I've fished it with others so I'm not sure about access but it winds its way through much of SW Ohio and there are canoe rental place all up and down the stream where you could rent a canoe to float fish the stream too.
About a 2 hour drive west of Dayton is Brookville, Indiana. The tailwater below Brookville Lake has some nice trout fishing. Typical Browns and Bows and typical tailwater (take a lot of #20 midges, scuds, and little nypmphs and some hoppers and ants in the summer). There are some big (18"+ browns) in the stream but the typical catch is in the 12-16" range.
Again, I'm not terribly familiar with fishing in the Dayton area but there should be plenty of places to wet a line in the area.