D&D is not what you want if you're looking for lighthearted.
There are a lot of options. For something lighthearted as you said I would try maybe Cartoon Action Hour - http://www.zmangames.com/CAH/
Its a surprisingly good game that plays well but doesn't take itself too seriously. Used to be able to download it in PDF but it isn't on RPGNow anymore.
Another game I'm fond of is Eldritch Ass Kicking. This one was also PDF but doesn't appear on RPGNow anymore.
It's a cool game but I don't know how much dungeon you'd really need to play it.
http://www.mysticages.com/eak/
Theres a nifty game called Donjon where the players can make rolls to decide what appears in the dungeons they are going through. Could be a lot of fun with Chunky Dungeons. I bought Dunjon a while back but have yet to try it. Theres a monster book available as well. Looks like the price dropped as well.
http://www.rpgnow.com/product_info.php?products_id=294&
If you like anime then you might want to try out Big Eyes Small Mouth (BESM) from Guardians Of Order. You can download the Tri-Stat rules system from them for free too. Very cool.
http://www.guardiansorder.com/
Deep 7 (www.deep7.com) and Politically Incorrect Games (http://www.pigames.net) both make rule light systems that I like a LOT but they're a little lean on fantasy setting.
And last but certainly not least there is Savage Worlds. This is the best multi-genre game out there IMO. My favorite of the bunch, its extremely flexible and has the best community of gamers behind it.
www.peginc.com
Anyway, thats off the top of my head. Have fun.