I feel i should warn you; at level 1 a Housecat can be a serious threat. If you're a Wizard and you come up against a housecat, run, because it will win. If you're a fightery type (Melee Cleric, Barbarian, Fighter) then you'll probably come out on top. Anyone else is about 50/50.
Kinseti said that RP actions, and doing clever things like sneaking about and avoiding conflict will also net us EXP.
Neglecting that though, would you suggest we swat horse flies or something for experience?
Unarmed Kobolds\Goblins and Small Monsterous Spiders are the standard fare for 1st level characters. Watch out for the Spiders (they do have venom, it's weak but dangerous), and if you get any bigger than Small then get the hell out. With the Kobolds and Goblins same thing; watch what they're wearing though. If one is wearing nice armor and wielding something more advanced than a pointy stick he probably has class levels and will whup your ass.
If you're feeling brave you can probably mob an Orc or two.
Oh, and if you end up indoors, keep an eye on the roof. There's a low level critter called a Darkmantle which is basically a flying carniverous stalagtite, while not particulary dangerous, they're difficult to spot and if one plops onto the head of a squishy the squishy is probably toast.
Also;
always check for traps. Traps in 3.5 are usually 1hitkills or 1hitfuckyouups, you get one save, fail it and you're screwed (and possibly the entire party is screwed), so
always check for traps. Cursed items are less likely unless the DM is a dick, but unfortunately there's nothing much you can do about cursed items anyway (the only surefire way to uncover curses is a high level spell you won't have for ages.)
Now that i think about it; it's like Nethack or Adventure Mode. Play low level DnD the same way you play Nethack or Adventure Mode at low levels. High level DnD is an entirely different ballgame, but you don't need to worry about that yet.