I know I said Classic Traveller is game I wish I was playing in the Day One Post, but that's not why I think it's perfect for two hour session. I've played quite a few two-hour sessions and with Classic Traveller using the 76 Patrons adventure book.
76 Patrons is basically 76 adventure hooks with important features randomized on a D6. It could be a twist, a betrayal, a change to the maguffin. It doesn't matter, it means there are 76 times 6 (456) potential scenarios in that little black booklet. Each page has enough information laid out to improvise a good adventure scenario. They can be completed by a motivated group in less than two hours unless the GM pads them out with extra detail or complications.
Traveller itself allows for pretty much any kind of play. The galaxy is consists of endless isolated planets full of mystery and opportunities. The party can encounter any level of tech imaginable so there is no itch that can't be scratched in a short game.