A camping forum for VW bus and camper owners.
Let's get this out of the way right off: Lewis and Clark never visited the caverns, but they did pass through what is now the park.
On I-90 at either exit 256 at Cardwell (if you're Eastbound) or exit 274 at Three Forks (if you're Westbound) you can take a break from the interstate--and avoid climbing a big hill besides--and visit Lewis & Clark Caverns State Park, Montana's first state park. Open year-round except Thanksgiving, Xmas, and New Year's Day.
The park is essentially two things: the campground and visitors center at the bottom near the highway and the river and the caverns and visitors center (including a modest cafe) at the top. A beautiful winding drive with lots of pullouts and picnic spots and hiking trails connect the two. There are rattlesnakes around, so no turning over rocks or sticking hands in holes.
If you're just looking for a few hours' diversion (and want to avoid that big hill), a cavern tour should be on your to do list. It lasts about two hours and mostly trends down, the worst part being the walk to the cavern entrance. Inside, there are lots of stairs and even a slide. The ceiling gets pretty low through some passages and the floor is wet in spots, so hold on to the littler ones' hands. Tours are guided, cost $10 for 12+ and $5 for kids 6-11, and are definitely worth it. It's the second-largest cavern in the US, after Carlsbad!
If you're overnighting it, the campground is a level, fairly spread-out affair with not many trees. There are centrally-located flush toilets, coin-op showers, and a dishwashing station which makes it almost feel like cheating. The facilities are well maintained. Also right in the middle of all the camping loops is a nice and big, and new, playground. My daughter gives it two thumbs up. Cabins and a tipi are also available. Some sites are reservable, some aren't.
Most Fridays and Saturdays there are programs at the amphitheater or lower visitors center or both. Schedules are posted at the bathrooms and online.
We went the L&CC twice during the Summer of 2014 and really enjoyed it both times.