I'm not sure about the crystal case (I'm not even sure what you mean by watermarks... how is it getting wet? Do u mean sweat?)
But, if i were to look for an anti-glare screen shield, i'd look on
Play Asia or ask around in a local retailer like EB or Game Traders.
As for games, there are heaps i could recommend, but some of my favourites are:
Planet Puzzle League
Elite Beat Agents (and it's Japanese counterpart Ouendan 1/2)
Ninja Gaiden Dragon Sword
Contra 4 (this is quite hard though!)
New Super Mario Bros
Mario Kart DS
The Legend of Zelda Phantom Hourglass
Metroid Prime Pinball
Castlevania Dawn of Sorrow (or it's sequels Portrait of Ruin and Order of Ecclesia, though i haven't played them, i'm sure they'd be good)