We're playing with 2 players sometimes 3.
I've heard Gloomhaven is a 6 hour long game and that kind of frightens me. I don't think I'm ready for that sort of commitment yet.
While I haven't tried Jaws of the Lion yet (waiting for those reusable stickers!), Gloomhaven proper takes awhile to sort the pieces for the first time, but set up for a scenario shouldn't take much over 30 minutes when you're familiar with the pieces. As others have said, the actual game can take between 45 minutes and 2hours, which is still an investment but not 6 hours. That said, there's a bit to keep track of in Gloomhaven, so I might save it until you cut your teeth just a bit more.
If you're looking for some games that are quick to set up, I'd highly recommend
Cairn (2 player game that's a bit like chess). Each player has 5 figure pieces, there are a few cardboard tokens to sort (less than 20), and then you're off! Each time a player takes a simple turn, they're choosing an action token (say, move vertically/horizontally), and flipping it to reveal a different action (in this case, move diagonally). Earning a point also means placing a new space on the board with it's own unique rule you can activate if you're standing on it. So the board state of the game is changing, but the variables are kept pretty straightforward and simple. It's good, quick fun. Generally a 30 minute round, and pretty quick to learn/teach thanks to a small rule book.
If you want something that has similar fun takes on evolving play space, but would be good for 3 players who want something directly competitive,
Inis is a fun, odd game that you should look into. Like Cairn, you're essentially setting the game up just by taking the basic pieces out of the box. The rest of the board is built over the course of a game as random tiles are drawn when relevant. It's not exactly a war game . . . but it's in that vein. I highly recommend checking it out and it's designed by the same guy who made Cairn. Some of the sensibilities are similar, but they really play different because their end-goals are so different.
We also really like
Whitehall Mysteries: a Hidden Movement game where one player is Jack the Ripper and the others are a set of investigators trying to find him. Jack starts the game by picking 4 locations on the board on in each quadrant, where he'll drop off evidence. He has a limited number of turns to make each drop, and he has to drop them all without getting caught. The investigators are of course trying to find his trail, but it tends to be effervesent. It's a thrilling cat and mouse chase where as Jack, you'll go from laughing confidently at the investigators because they're 5 turns behind you, to panicking because they're one circle to your left and damn it, how can they not know where you are? As investigators, some stretches might seem like grasping in the dark, but when you find the first thread of a trail in several turns, that feeling is electric. The number of investigator characters never changes (always 3), but the game plays best with 3-4 total players so that Jack can listen into their plans. 1v1 is fun too, but it's often a bit quieter and cerebral.
Oh, and engine builders like
Wingspan and
Everdell can be a lot of fun. Because these games aren't directly competative (you really can't screw anyone over), they're great for players who are just getting into more complex games. In both games, you're drawing cards and using the actions on them to, hopefully, create chain-effects that let you play bigger, better cards. Because they aren't so competitive, you're more competing with yourself and the options you're given in any one turn. There's always a satisfaction in making the choice that seems right to
you, and the number of choices is simple enough (especially in Wingspan), that deciding "This is probably best for me, right now" is always possible, but rarely a "sure thing." They're different in a number of ways, but you really would only need one for a bit since they overlap in some key ways.