If it is open lines will depend a lot on the weather. On our last visit there (beginning of June) we went on a Saturday expecting long lines. However, we found the lines for the coasters to be quite short, only 15 minutes or so. The line to get in the waterpark was very long as the temp. hovered around 95. Also the line for Gully Washer was about an hour. You will need to do a water ride at some point probably to cool off, but get a lot of rides in on the coasters in between if it is like when we went. I would hit Poltergeist first, then cut over to Superman and get as many rides as you can, this coaster is awesome. Next go to Rattler if you want to abuse yourself, or need to add to your count. I've rode it once and that is enough. Next, is Roadrunner Express, one of the smoother mine trains I've ever rode. Last, for coaster I would hit Boomerang. It is one of the smoother boomerangs, but you've probably been there-done that before. As for flats SCREAM is fun but will likely have a fairly long wait regardless of the lines for the rest of the park. There is also a Frisbee, Top Spin, and Chaos for flats. We didn't try the Scooby Doo dark ride last time because it had long lines, but it is right next to the Frisbee if you want to give it a whirl.
I'm not sure what the status of the park is going to be on Friday though. More strong storms were supposed to hit today and Tuesday. We planned to go to Astroworld today, but went to SFoT instead because of the storm front across basically all of S. Texas.