It might be at the outer limit of your traveling, but there is a very nice botanical garden near Buchanan, MI. It's called Fernwood Botanical; I'll put the e-address here incase I don't do the link right: http://www.fernwoodbotanical.org/. There's a smallish arboretum there, 5 acres if I recall. And many, many more acres of walks - woods, streams, prairies. They also have a small indoor fern greenhouse with some neat species.

Here is a link that might be useful: FernwoodBotanical

I also just thought of the Fredrick Meijer Gardens in Grand Rapids, MI. They have several largeish greenhouses; an arid room, a tropical room with orchids and waterfall. Outdoors have never seemed terribly spectacular to me. Though they do have a swamp walk and several perennial boarders. What does make it interesting is the art. Evidenly Mr. Meijer appreciated art and has many statuary pieces incorporated with the gardens. The most renowned is his reproduction of DaVinci's bronze horse. Not something to miss if you're in the area.

Hope you have a fun trip. I've looked at the boat ride myself. I have a friend in the appleton area and we're looking up all the great arboretums and gardens on your side of the lake. Seems like there's too much over there to see for the short trip that I'm able to take.

Thanks for the info!