Cowles Bog is one of my favorite places for a spring hike. While not a true bog, (the wetland it's actually a fen), it is home to a variety of plant species that you don't see in too many other areas of the dunes.
In May, thousands of ferns unroll into plants with fronds over three feet long. In this photo, they're not fully opened, so they give an almost prehistoric look to the wetland.
In addition to the variety of wetland plants and animals, Cowles Bog contains a number of other types of landscapes including prairie, forest, beach, dune, and savanna. A two mile hike from the parking area to the beach will introduce you to most of these environments. For a bit of variety, walk along the beach to find another trail head, then follow that back to the parking lot.
Worth a visit in any season.