Do you already have jEnesisDS and NeoDS? They're both amazingly high quality emulators (for Sega Mega Drive/Genesis and Neo Geo respectively). Jam Sessions is a fun time waster if you like making music; not much of a game so much as it's a virtual guitar. It has a recording function, but I have no musical talent whatsoever, so I have fun just sitting around strumming. If you're interested in learning Spanish, My Spanish Coach is actually remarkably fun. It's mainly just vocab, so it won't help you much in a conversation, but as a game which happens to be educating, it's surprisingly entertaining. Get the (U) version, the (E) version is split into two separate games. Uno/Skip-Bo/Uno Free Fall has a pretty good implementation of Uno, could look a bit nicer and be a lot more customisable, but it's fun for what it is. Uno Free Fall is a crappy puzzle game, and I've never played Skip-Bo, so I can't vouch for that. WordUp! is the best homebrew game I've played for the DS, period. You spell out words by dragging the stylus across a grid, and the letters disappear when you use them. If not for the default Palib font it uses I could be fooled into believing it's a commercial game.
On the GBA side, Astro Boy - Omega Factor is a game I sorely regret missing out on at release time. I only recently caught up with it, and it's fantastic. If you're feeling lazy, play it on easy, it's still an enjoyable game and remains a challenge in places. bit Generations games: Dotstream, Dialhex and Orbital are the best, and they are some of the most fun I've ever had on the GBA. Dotstream is a racer with powerups but you play as a dot, and can slipstream the other dots to gain speed. Dialhex is a "match the pieces" puzzle where you rotate hexagons. Orbital is like Katamari Damacy in space; you start as a planet and must collect satellites and absorb smaller planets until you're large enough to have a star orbit you. Very slow paced, very brilliant. SuperWings is a funny, difficult homebrew scrolling shooter. It won one of those contest things on PDROMs, classic graphics and gameplay.
Hope some of that stuff is new to you, enjoy the holiday.