The Indiana Dunes State Park has several really nice trails all in a compact area.  The park is very popular, and the line of cars to enter can be upwards of an hour in the summer so it pays to go early.  Of the parks 10 trails my favorite by far is #2, starting out from the nature center, it loops around a marsh/swamp, until it intersects with trail #10 which I take back to the beginning.  It is a great trail for wild flowers, fungi and lichens, all topped off by a half mile long boardwalk across the marsh/swamp which is nothing short of awesome.