Critical needs are probably going to be a place to stay for the flooded folks and their pets. If you can offer that, you've solved a lot of problems right there.
Physical help at the house is awesome. There's no way all of the packing up, throwing out and tearing out can be accomplished without a lot of boots on the ground. Everyone needs to know that whatever they wear will probably be ruined for future wearing out in public although it can be washed and worn again day after day to work in the disaster area.
Food and drinks for people who are working are invaluable - water, cokes, brownies, cupcakes, fruit, nuts, chips or whole meals. It all helps. But check first -- better to have food every day instead of lots of food some days and none on others. There probably won't be much other than ice chests for storing it.
Loaning Ice chests, utility tables, lawn chairs, fans, furniture dollies, etc...
I still had a bunch of stuff in the carport to clean and pack when we had everything out of the house and were done with the tear out. Most folks figured I was done by then, so I really appreciated the folks who came and helped clean furniture and pack stuff those days.
Gift cards are great -- for restaurants, Home Depot or Lowe's, pet supply places, etc. It was nice to be able to grab a meal someplace without thinking I should really be saving the money for the house. And I long ago lost count of how many trips I've made to Home Depot and Lowe's and hardware stores.
I had some friends who came and got most of the clothes from my closet that didn't get wet but smelled like all holy hell, and they washed clothes for me. Huge help!
Meals - friends fed me home cooked meals for weeks; they might bring a plate and leave it for me or invite me over for dinner. Most days the plate was better for me because I half killed myself working at the house and was too tired and dirty to want to go to someone's house for dinner, although on the light days (with a couple days advance notice), that, too, was great.
Pray for their patience to hold out. I know lots of people had to be praying for mine, cause there's no way I could have gone that long and through that many trials and tribulations without a Vesuvian-style eruption absent divine intervention!
And last of all, remember that they'll probably be able to use a little help putting it back together, too!