I've been to the Badlands several times, the best viewing is at sunrise and sunset the colors are amazing! And the temperatures during the summer are more forgiving early and late in the day. Summer months can get very hot midday. We were lucky we visited during overcast and rainy day this last trip it was perfect. We did a driving tour this time, but there are several hikes you can take if you have more time. We enjoyed a picnic in the park there are two different locations you can picnic they are shaded but on a hot day you probably wouldn't find much relief under them. Since there aren't too many trees around. Badlands is a must see stop on anyone's itinerary visiting the Black Hills.
There is lots to do in Custer state park from paddle boating to swimming in the lakes to horseback riding. We enjoyed seeing the buffalo roam around, and hiking around legion lake, and roasting marshmallows at our modest cabin within the park.
Great little museum, but there wasn't much for picnic spots and the Rangers weren't very friendly. It is a good quick stop on your way to break things up. Our kids enjoyed unloading and loading the mock wagon and getting their park passports stamped. Scottsbluff National monument down the road had more for the kids to do with a Junior Ranger program and etc.
Judy was a great host and had some fun UFO stories, my 8 and 4 year old enjoyed walking around. I am not sure I would camp there, but it was a great photo op stop. You are charged $5 per car load or $2 per person. Bring something along to leave "your energy" in the garden.