Charlotte area local here. For parking, don't bother with Bruton Smith Boulevard and the main lot, traffic can be a bit of a mess. What you want to do is, assuming you're coming from the south from Charlotte and the airport, go one exit past Bruton Smith/Concord Mills Boulevard on I-85 and get off at Poplar Tent Road. Turn right, and then turn right again at the first road onto Pitts School Road. Take that little two lane road a couple of miles and then turn right onto Concord Parkway to head towards the track. Once you can see the NASCAR track, as soon as you go over the little bridge, turn right and go up the hill. You can park for free in the area behind the backstretch of the dirt track. There should be someone at
this entrance that can scan your tickets and let you in without having to walk all the way around to the main gate by the tower. As for rain, I've never had an issue with flooding or anything like that. There's a creek that runs around that part of the property so it drains well in my opinion.
As for potential rain delay activities, the area in and around the mall on the other side of the interstate has pretty much all you could want. Tons of restaurants, everything from fast food, breakfast places, sports bars, steakhouses, whatever you want. Also an aquarium, movie theater, and go kart track there to kill some time. There's also a couple of NASCAR shops nearby like Hendrick, Trackhouse, and Roush, although they may not be open for visitors on weekends so check before you stop by.