Every man wants to be able to have a space to call his own. Somewhere where he and his friends can hang out, do whatever they want, and have fun doing it. Caving in and allowing him to do so is much easier than worrying about him going elsewhere to do the same thing, and can be a lot of fun to customize just for him. There will not be a lot of furniture required, just some key pieces that you will know he will need, and some creative decorating.
For The Sports Fan
Some guys never outgrow their love of sports, especially if they played a lot when younger. For them, you will need a big TV, the better to see all the plays with. You will also need someplace to hold all the electronics, so invest in a good solidly constructed unit, like those found in the lines of Amish entertainment centers. These will be perfect to hold all of his home theater electronics, and all the remotes that come with them.
Sports fans need food and drink, so investing a bar for the man cave might be a good idea as well as some extra tables and chairs. Pub tables work great for this, especially since they will probably be jumping around a lot during plays. Make sure that you have at least two small refrigerators under the bar, one for beer, and the other for soft drinks and snacks. A microwave would probably work well, too.
The Game Room
Guys who like to have game rooms set up as their version of a man cave probably miss having that special hangout place of their youth. Whether it is in a basement or in the garage, you will need plenty of tables, chairs and game related stuff to keep them all happy over a long weekend. Televisions can be included, especially if you include some of those Amish entertainment centers for their storage capacity. Plenty of room to stash video game consoles, and the games, and videos for a long weekend of hanging out.
A pool table, dartboard, or foosball table to keep them happy while waiting for their turn on the video games would be nice. Refrigerators to stash snacks and drinks, plus a microwave or two, and some extra game tables. Other than that, plenty of soft chairs to crash on for the weekend should do it.