Dangit, because of my newcomer status, I'm still uncomfortable about posting :|
Though you say you're into RPGs, I'm just gonna go and suggest whatever. These games are all highly addicting, and can be considered casual.
PICROSS. YES.
Picross (aka nonograms) is, like, the best puzzle game ever. If you ever liked newspaper puzzles, this is a must-have.
Now, if you don't like those kind of things... Go broaden your horizons and buy this game anyway.
This game makes Sudoku look boring, and this is coming from someone who used to be addicted to it.
If you can find one online, I highly recommend METEOS. YES.
This is a falling-block puzzler. Meteos is to Tetris as Picross is to Sudoku. That is all I shall say.
The Professor Layton series is another great one. It has a good plot, nice music, wonderful animations, and plenty of riddles, brainteasers, etc. You may also want to get Rhythm Heaven, but I don't have much to say about this one. It's just addicting and wonderfully satisfying.
Oh, and of course I must finish with this: Kirby Canvas Curse and Kirby Super Star Ultra. Because Kirby is awesome.