I'm partial to OH because I lived there for almost 40 years. Not sure from your post what kind of plants you'd want to drive around looking for? Since that's probably the closest other state, probably do-able for you. Assuming things in these places are still as good as decades-old memories...
A couple of really famous places are in OH, Glasshouse Works in Stewart, famous for stocking arcane plants, especially tropicals. Primarily a mail-order source, but they seem to welcome visitors as well. I'd call first, and have read very recent anecdotes about people visiting there.
http://www.glasshouseworks.com...
Baker's Acres in Alexandria is a major breeder of new Coleus, and their store is a huge, landscaped complex, a family business. The kind of place it takes hours to visit. (Don't miss the bathroom!)
http://bakersacresgreenhouse.c...
(Click on 'download our famous catalog' for a fun read! They shut down for winter.)
Baker's Village Garden Center in Powell was always a thrill to visit, beautifully decorated upscale place, plants you've never seen before probably, large, vigorous specimens of tropicals, annuals, perennials, but very pricey.
http://www.bakersvillagegarden...
DeMonye's is near the Columbus airport. They are known for having flats of starter-size perennials, a huge annual sale on annuals, much wider selection than any BBS.
http://demonyes.net/
I always had high hopes for finding something new when visiting Oakland Nursery:
http://www.oaklandnursery.com/
Wow, apparently there's several stores now.
Dill's in Groveport, I never left there without something new:
http://www.dillsgreenhouse.net...
The *only* mom'n'pop garden store (that has unusual tropicals and landscape plants, not just trees & shrubs) within 2 hours of where I live now is Dothan Nursery in Dothan, AL. They also have a huge section dedicated to mini/fairy gardening. Never seen anything like that before, but haven't been in another real garden store in a decade, except one in New Orleans last year.
http://www.dothannurseries.com...
If I ever get to take a plant-road-trip, it would be to Apopka, FL!!
