Park may or may not be crowded on Monday (most likely not). I know some schools have started, but most haven't. I don't expect it to be too bad though. NTAG and La Vibora always have the longest lines. I recommend hitting La Vibora as early as possible, because as was said it Does keep a LONG line all day. Surprisingly Batman was a bit longer when I was there a couple weeks ago as well, almost an hour I think, but I believe they had been having maintenance issues that day. Both Batman and Mr. Freeze are usually more like 25-30mins on a weekday, at most 45mins I would think. Flashback, Judge Roy, Mini and Big Mine trains are always walk-on's (so glad they quit "double ride" Monday's for those four, made the lines for those Long, most people get two rides on them any day anyway without the hassle...) The best time I think to ride NTAG is later in the day when it gets darker, the line is usually shorter by then and it makes the three tunnels 10x darker and the lights in the "honky tonk" tunnel more impressive. Also Please don't forget water!! I noticed most, if not all, of the food vendors had cups of it iced for people to get for free. There's also several little air-conditioned ice cream shops that, when not crowded, will let you take a little sit down break inside.
Also, to the person asking about kids rides:
The Yosemite Sam boat ride hasn't been mentioned and it is fun, good for little kids, but still enjoyable for adults (plus indoors to get out of the HEAT)!