Nintendo Land is a fun one, especially when played with other people. Of course, I realize it comes with several Wii U bundles. But hey, in case it didn't, used copies tend to go for under $10 these days. I got mine at a local used game store for $8-$9, and it looks brand new.
Also, I bought Gunman Clive HD Collection off the eShop, and I've been enjoying it so far, too. Good bang (ba dum tss) for your buck.
And of course Super Mario Maker and Smash Bros. are fantastic.