Here, let me make you a short list of must-plays for the spin-offs. If you're ever going to look into any of them, these ARE the ones to get. Anything else isn't worth your money (though, I have enjoyed most of the spinoffs).
PMD-Explorers of Sky: If you didn't play the first two (Explorers of Time and Darkness), get this one. It follows the exact same story, but has a bunch of expanded features that easily makes this the ULTIMATE of the PMD dungeon-crawlers. You may have to check used game stores for old DS games, but if you find it, buy it on the spot! The graphics may look silly compared to the 3DS's norm, but this is definitely a must-play still.
PMD-Blue Rescue Team: If you can find it and cheap, get it. This was the original PMD game before the battle mechanics transitioned to the 4th Gen's. The story is absolutely charming, and the 3rd Gen gameplay is really easy to exploit for quick play-throughs. I've easily put over 500 hours on my copy, and I've replayed at least ten times since 2006.
Pokémon Conquest: I know I mentioned this one before, but this easily one of the best strategy game you can get for any system. You may find it awkward that all Pokémon are restricted to one move/ability, but amount of strategy you can put into exploiting them to your advantage. The main story is actually very short, but you'll find yourself putting in a lot of time doing the post-ending missions that you'll unlock. That's the best part of this game, though. You actually get to play as characters other than your default "Warlord," and the game gets insane from then on because rival nations will actually start attacking you and expanding their own armies to conquer your kingdoms (that's a synopsis of the entire game).
Pokémon Ranger (THE ORIGINAL): I'll be honest, you'll end up destroying your 3DS's touch screen, but this is easily the most addictive game in the entire subseries and the hardest since, if you mess up during a battle, you have to reengage the entire fight again.