The bad news is that aside from Heaven Hill's Bernheim plant in Louisville (which does not offer tours), none distills year-'round -- and summer is the off-season.
That said, the two most scenic are Maker's Mark and Woodford Reserve, both located in the countryside in lovely settings.
While Buffalo Trace is historic, has very friendly folk, and produces probably the best stable of premium bourbons (think George T. Stagg, the Van Winkles, Blanton's) today, its standard tour isn't that great unless you can go during distilling season, when you're more likely to see activity around and between warehouses. They offer a 'hard-hat' tour during distilling season which is well worth the trip, but it's not distilling season.
Heaven Hill's Bourbon Heritage Center is a fine exhibit, but you don't see anything of the bourbon operation itself except for a single warehouse. You do, however, get to sample the Evan Williams Single Barrel and Elijah Craig 18yo at no cost (except on Sunday, when they can't serve). It's a convenient waypoint between Woodford and Maker's Mark (try the bourbon balls at all stops!).
Skip Beam's "American Outpost" -- it's little more than a gift shop located next to the distillery. The tour consists of Jim Beam's former house and a peek into a warehouse.
Wild Turkey also is pretty mundane unless you happen to catch Jimmy Russell for a tour guide
. Nearby Four Roses is worth a look, but call ahead -- they've only recently begun public tours, and the schedule is spotty.