I understand the convenience and safety of the changing area on the floor and encourage that if you can (if you are ever tempted to turn around from your baby when they're up on a higher changing surface even for a second to just grab something, etc., DON'T!--they move so quickly!! We never had a fall but I know so many who have...makes me shiver just thinking about it!!)
That said, both DH and I have really bad lower backs and stooping to change just isn't comfortable for us. We keep one hand on baby at all times but use a higher changing surface--any surface will do, just keep it covered with something washable (I recently posted a pic of mine in the Crafty Mama's thread). You don't need a fancy changing pad, just use a towel, or two etc. You can add a folded up towel or washcloth to make the head area more padded. I always keep wipes on hand (we use cloth so I also have a water source--squeeze bottle, etc.), whatever diaper cream or baby balm I may want (I don't use it unless baby seems to need it and with DD, with frequent changes, we hardly ever needed it), of course, your diapering supplies. I kept an empty disposable plastic food storage container nearby (without the top) as a place to park the dirty diaper (a 'diaper dumper!') until I could attend to it (cloth may need to be sprayed before hitting the pail or if you're using disposable, you'll want to dump out the poo once it's solid enough but regardless, rolling it up to contain the mess before tossing it might take two hands and will need to wait until you're done with babe up high). I also keep an extra diaper or cloth nearby to cover baby until I have the clean diaper all positioned...in case the baby pees (etc!) once fresh air hits but before you've re-diapered. Keep a spare change of baby clothes handy too because poop travels. Anything you can keep right there to keep from having the urge to leave baby and grab anything. If you suspect a severe blowout (common when baby is growing and your diapering sizes need to upgrade), just head straight for the bathtub!
For nursing, I always wanted water (LOTS of water!), a snack, pillows to support my back and baby (whatever nursing type pillow works best for you or just several smaller pillows to support your arms, etc.), my lip balm, a book, a footrest, the phone, and something to write with/on (I'd jot down little notes I wanted to remember about baby or whatever...grocery list!), a burp rag or two, and sometimes a blanket for me and baby blanket in case it was chilly--once we were parked, I didn't want to get up for a while if I could help it--DD was a long nurser! I learned to hit the bathroom for a potty break before sitting down too though I have to admit to carrying a latched/nursing/sleeping baby with me to pee when I couldn't heart waking her up or couldn't wait.