Regarding Spelunky: As much as it's a fun game, I wouldn't recommend it as a "gateway RL", it being a realtime platformer, and all. Liking Spelunky doesn't guarantee that you'll enjoy Nethack.
brogue is probably a good suggestion. Depending on personal tastes, Legerdemain might be worth considering. The interface is not too difficult, it has tiles and a nice setting.
Some quick games with a punchline: Teemu, Vicious Orcs. Lastly, let me mention that my own game Squirm, for all it's shortcomings, was developed as a coffeebreak RL with new players explicitly in mind.
As always,
Minotauros