Here's the screen with the Aerobics choices. Reading down from the left, there's Hula Hoop, Advanced Hula Hoop, Rhythmic Boxing, Basic Step, Advanced Step, Free Step, Basic Jogging, Two-Person Jogging, and Free Jogging.
For Hula Hoop, you stand on the balance board and swing your hips in a big circle. Every so often, one of the other Miis tosses you a hoop, and you must lean over to that side with your arms extended to catch the hoop. You get points for every circuit multiplied by the number of hoops you're spinning at the time. When you go really fast, the hoops are up around your neck. When you slow down, as I did to take this picture, the hoops start slipping down to your feet.
Advanced Hula Hoop is similar, but a bit longer, and you do half in one direction and half in the other. You also get to see a tracing of your weight movement for each direction. In my case, one direction is notable less smooth than the other. If you do this quickly, it is a bit aerobic, but it doesn't last long enough to really be a workout. It does really use your torso muscles, though, and I can't do it too much right now without feeling rather sore.
Rhythmic boxing is also not terribly aerobic, but does give your arms some exercise. And it's the activity that makes me understand fully why they keep warning you to make sure your wrist strap is fastened, since you're throwing punches directly at your TV screen. It was hard to get a good picture of this one, since it's very active. You hold the WiiMote in your right hand and a secondary controller (the Nunchuck) in your left hand. The little guy on the right demonstrates a sequence, and then you do it yourself on the big bag. You step forward off the board each time you punch, and then step back to block in between. A target comes up when you're supposed to punch, and a big splat graphic with sound comes when you connect. It actually can be rather satisfying.
At the end, you get a few seconds of free-style rapid punching that if you do it with vigor, causes steam to pour out of the punching bag and then it blows up.
Jogging is a lot of fun because you job around a virtual island, complete with a town, a waterfall, various types of bridges, etc. You follow a randomly-chosen Mii (different each time), and you have a choice of 3 different routes of differing lengths. And you have people cheering you on as you go.
You don't use the balance board for this, you just run in place with the WiiMote in your pocket or held in your hand. The system uses that to tell your speed. Your Mii's head bobs up and down in time and you hear your footfalls, which even change in sound based on the surface you're running on. Here are a few pictures of the short run.
Your Mii stretches before the start:
You pass a waterfall. (In this picture, my companion has turned around and is waving her arms to urge me to get going, since I stopped to take the picture.)
Through a rock formation. Note that the symbols at the bottom show that I'm about halfway through the course.
It's hard to see here, but every so often a dog comes up and passes you. (He's the brownish blur in front of the little house.) And I have discovered a trick, which is that if you speed up when the dog goes by, you sometimes can switch the arrow so that you start following the dog and get to see a different route.

And here's the finish line for the short jog. When you cross the finish line, your Wii jumps around and celebrates with the other Miis.
As I mentioned, you get to choose between 3 lengths of run, each with a different course. The longest one takes you up to the top of the waterfall.
I haven't tried the two person run, but I assume that's used when two people want to work out together.
The free run is when you want to do a longer run - 10, 20, or 30 minutes. You can stay in the game and circle around the island, or you can choose to switch the TV to a different channel and watch something else while you run. You could also use this in conjunction with a separate exercise device, such as a treadmill, if you have one. The neat thing is that there is a speaker in the WiiMote, so you can get progress messages and time checks as you go.
I haven't quite figured out the run scoring. For the Basic Run, it seems to be based somewhat on the consistency of your jogging, not just the speed. But for the Free Run, it does seem to go by speed, and shows you as going further around the island the faster you run. And it gives you a score for the number of miles covered.
Finally there is the Step Aerobics. You know what this is like if you've ever taken a step class - you step on and off the balance board in ever more complicated patterns. You can either watch the scrolling diagram, which uses color to indicate the different step patterns, or just watch the Wiis to see what they're doing - which is also useful to see what you should be doing with your arms.
Like the jogging, there are several versions: a short, simple version, then an advanced version with faster, more complicated steps, then a free version where you can switch away from the screen. In the free version, you can choose to have the WiiMote give you various sorts of metronome-like sounds if you wish, and can adjust the speed to suit your pace.
Scoring for the first two variants is based on how accurately you are in time with the beat. Each step gets scored as either missed (0 points), Good (1 point), or Perfect (2 points). I find it really hard to get the Perfects and find that only about 1/4 to 1/3 of my steps get scored Perfect. For the free step, like the free run, you are just scored on how many steps you get in.
The balance board is not as high as a typical step board in an exercise class, so I'm thinking that when my thighs get a little stronger, I'll maybe slip something under it to raise it a bit and get a better workout. Right now, though, I'm fine where it is.
I like all of these exercises and do them all pretty consistently. It would have been really cool if they had a jogging version where you could decide which way to go at various turning points instead of having to follow a fixed course, but that may come in the future. I think that the Wii Fit has only scratched the surface of what it would be possible to do with this type of device.