Luigi's Mansion 2 MP is solely co-op based. You tackle 4 different type of missions, with varied difficulty and length and you can unlock an infinite length variant once you complete all 4 missions on the hardest difficulty and longest length (25th Floor Thrill Tower Expert).
As for what to buy on the 3DS, I personally recommend Pokemon Mystery Dungeon: Gates to Infinity. While it bears the Pokemon name, it's a spin-off from the series and has a Dungeon Crawling mechanic that similar to Action-RPGs such as Diablo and whatnot. It's a pretty fun game.
Monster Hunter 3 Ultimate is also a good choice if you're up for challenging games with a lot to do. And when I say challenging, it really is, as the MH series is not new-comer friendly, but it's pretty interesting once you get the hang of it.
Those would be my suggestions, as I have yet to download Donkey Kong Returns 3D (will do it next month).
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
