A lot of the Pokémon spin-offs are really good.
I personally recommend the Mystery Dungeon series, namely Explorers of Sky and Blue Rescue Team. They might come across as childish at first, but make no mistake, these games can be both insanely challenging (I'm looking at you, Purity Forest) and are loaded with content. One does not beat Blue Rescue Team in a day. Absolutely beautiful stories, too.
Ranger: Shadows of Almia is pretty good too. It's the second entry to the Ranger spin-off games and, IMO, the best by far.
Ōkamiden is fantastic. Play it and play Ōkami.
And if you like Zelda, try Phantom Hourglass and Spirit Tracks. Both get some flak but I found them fun.