Wii Sports is a party game that is surprisingly a good pack-in with the Wii.
My personal favorite is boxing, where the goal is to knock your opponent out. Single player is single screen, where the game pits you against AI fighters that get increasingly difficult the more you climb the ranks to become champ! Multiplayer is only for two people at a time and the goal is the same, knock the other fighter out. The punches are easy, except for the hooks, and they leave you vulnerable for a big head or body blow. The timer ticks down the three rounds, but don't worry about that, as neither of the fighters has very much stamina.
Baseball is a true classic, although it does lack more effort from the player him/herself. Either you bat by swinging your remote, or pitch by going through the pitching motion. Hold some buttons to make a couple different pitches, to make it more difficult to hit. The game determines how many bases you run by how long it takes the fielders to get the ball.
Golf is simple, just swing the remote like you would a golf club, taking care not to swing too hard, and watch the wind! The hills on the green add a little challenge, and there are holes that requires precision is your swing, like going from riverbank to riverbank to the hole.
Bowling is one of the better ones, hold the remote in front of you like you would at the beginning of your swing in real bowling, then hold a button as you approach the lane and let go right as you get there. A twist of the wrist will put some spin on your ball.
Last of all, tennis is a game that again, does not require much effort or skill from the player, all you you need to do is swing the remote, the game does all the running for you.
A nice feature to all the sports is a skill point system. All sports (except for bowling, which will give you them any time, and tennis, which will give them when players are on a team) will only give you points for single player mode. You can gain or lose points by how well you play.
Training can help you get better at sports you may not be very good at, but it could be a little more thorough.
Daily fitness tests are a good addition to the game, you go in with your Mii every day, (It actually doesn't matter if you don't come in every day), do some tests, and the game will give you a Fitness Age.
Overall, Wii Sports is a game that is good with multiplayer and single player, for a while.