Water is always a good choice for kids when it gets hot and they never seem to tire of it. We recently got a slip-n-slide that is inflated on the slide part, and then it dumps you into a shallow little pool at the end. Have contests for best slide, fastest slide, best belly flop, etc. We also had a water balloon toss contest that the kids really loved - and to get them to clean up the balloons on the ground, we bribed them with cupcakes as soon as they were done. Kids also love to dance, so some luau music with a dance contest might be fun.
As far as food - a traditional luau has a pig roast, so how about doing pulled pork samwiches - easy in a crock pot and then make some fried rice (Hamburger helper actually has a really good one that you add real egg to, but not the chicken). We also made fruit shishkabobs and everyone loved them, but they were timeconsuming to make - pineapple, strawberries, green & red grapes. Your daughter would also be able to help make these with you so she'd feel more invested in her party. Add a jello and a yummy green salad with maybe honey/mustard dressing and/or a cold bean salad (really good one on SouthernLiving.com with green beans, pimento & grilled onion) and then you'd be set.
Hope these ideas help! Just remember to have fun yourself!