Sonar is a game my wife calls "Battleship for Grownups". This gets the point across rather succinctly. It's a shockingly fun game with very few pieces in the box necessary to play. The basic setup is the same as Battleship, but there is only one ship you're commanding - a submarine. They recommend the game is played with 2 players per submarine. One person plays as the ships captain and the other plays as a radio operator.
A screen not unlike a dungeon masters screen separates the two teams. The radio operator and captain of both teams are each handed an identical map which shows the area where the submarine hunt will take place. There are four map variants in the box with more or less islands on them all the way to open ocean. Everyone also gets a dry erase marker to draw on their map. Further, the radio operators have a clear piece of plastic they can place over their map. This is used to record and predict any movements by the enemy submarine.
The submarine teams take turns with the captains calling out the maneuvers of their submarines. Each time you make a standard move, the submarine accrues one energy (to a maximum of four) which can be spent to engage a sonar ping, take a silent maneuver, or fire a torpedo. The nuances of the game are exposed when both subs are closing in on accurate positions of their opponents and tracking and carefully using energy is key to being in the right place at the right time to be able to fire off a torpedo.
I've had a ton of fun with Sonar. It's quick to teach somebody the rules and a great way to kill some time for four people. There is a more complex version of the game called "Captain Sonar" which I am unfamiliar with. That game supports up to 8 players and adds more roles to the game in addition to the captain and the radio operator. I'm not sure if I'll ever expand to that version, but give this a look if you want a casual to pick up strategy game with a surprising amount of depth.