My go-to for time killing is Star Realms, which is a competitive deck builder that can be played in person or online. I have both the mobile app and a very extensive library of the physical cards, thanks to a fortuitous buy on G+ from someone who was getting rid of some games.
The nice thing with this game is that, although they sell booster packs, the contents of each pack is entirely known. The core box, at $15, is all you need to play a 2-player game, though if you add a second core box, you can expand to 4. I think the maximum number of players is 6, though I’ve never played with that many.
There are a large number of variations on the game, and the company released a high fantasy variation called Hero Realms. Hero Realms is cool because there’s a campaign expansion where you play cooperatively against a campaign deck.