Hiking the Grand Canyon

I don't think you will have any issues getting a rim-to-rim permit in March, but I could be wrong. There is always the Hermit Loop Trail, too. Rim-to-rim can take as long or as short as you want as there are plenty of diversions along the way like Ribbon Falls and Plateau Point. Water shouldn't be an issue, so don't overload yourself. There will be places along the way to refill at the campgrounds, one of which I think has actual flushing toilets. If you have never hiked desert climates before, be warned that the water does evaporate out of you faster than you think.

When I'm on the North Rim, I carry a fleece sleeping bag liner to use if it gets colder than expected. They're about $10 on Amazon and work well as a pillow if you don't line your bag. (In the summer, that's what I'll use in a hammock instead of a bag.) You might bring a small black light to hunt for scorpions. I doubt kids from MS get to see them very often and it's pretty cool to see them glow. Lots to see and you're eyes/brain will have trouble taking it all in at once.