I've never personally done this but I have thought about it and it is a worthwhile idea. Car, train, and bus trips are all also ptoential playing areas.
Minis are, of course, right out so you'll need to get used playing without them if you're not already.
Dice area nother big issue. I was thinking a large enough, though not to large, tray with raised lip around the edge would work great for this. A cookie sheet might work, though that's a bit large. There are also those electronic dice rollers, though I don't think I've ever seen one other than a d6. If you have lap top with you you're set.
Really if you have laptop with you it can probably do everything you might need all by itself. You could even use, say, MSpaint or something to keep a crude battle map going.
Though it's not D20 I also feel I should mention
Sherpa which is generic little rpg specificalyl designed to be run outside or in other non-gaming-friendly situations, I believe it was originalyl designed for the author to use while hiking. It might have some good ideas you could plunder.
Scroll down, this is just the intro and the link to the actual text of the free version is easy to miss.