My son was on Risperdal for several years for Asperger's and we felt it worked well. Yes, at the time it was off label, as Sue mentioned. To be honest, we've used several meds off label for my son, with varying amounts of success.
Risperdal was good at minimizing the intensity AND frequency of my son's meltdowns. It wasn't perfect for him, but was better than what we'd used previously. Currently, we have excellent results with his meds: Strattera, Abilify, Clonidine and Luvox. One thing that is consistent with Asperger's kids-no two kids have the same issues OR get the same meds to work the same way!
Yes, we had some weight gain while he was on the medication, honestly, when we added Clonidine, he really packed on weight. We swapped to Abilify over the summer, and he's dropped about 10 pounds. With most of his medications, it has taken about two weeks to reach full efficacy. You'll probably notice a gradual improvement. For the first few months, most doctors want to see your child more frequently. I will chime in that until he was on Risperdal, my son stimmed a lot. Now that he's not on it, the stims are back, but not as much as before.
One thing that I've learned is to research as much as I can before starting a new medication. We've been extremely fortunate in that all the doctors (except the first one who refused to consider anything other than ADHD) have been extremely honest in what they think will work and what won't.
Regarding the parks in the summer, Sue also brought up my favorite thing-the WDW stroller. When a child on the spectrum gets tired from all the walking and the heat, NO ONE wants to be around for the resulting meltdown(s). The strollers save your sanity, close off three sides of all that stimuli and keep your child comfortable. We also bring a beach towel to drape over it (if needed) or use as a pillow and my son insists on a baseball cap. Normally, he hates them, but uses them in the park.
Schedule down time. It's hard when you don't make frequent trips to not try to pack everything in, but if you schedule day 3 or 4 as a poolside day, you'll get more mileage out of your trip. We also would decompress poolside at night a few times. There is something about the water that calms my son.

If your child has food issues, get a fridge in the room and keep some comfort foods (we had yogurt and juices).
Good luck with the medications and enjoy your trip!
Suzanne