I picked up the 3DS version of this yesterday, with a little trepidation, having found the London game to be even shallower than these games usually are.
The basic one player game, which is all I've played so far, lets you pick your event. There are quite a few events, and "extra" versions of all of those events.
I'm rubbish at 100m, so I could only come third on that, even with Sonic. The boxing was actually quite strategic. Playing with Donkey Kong I was up against some short but fast character from the Sonic series, who had lightning fast punches whenever I came near. Eventually I learned to keep out of his range and flattened him. Archery was great fun. Table tennis was basic table tennis, and the hoop gymnastics was the best game - it's a rhythm game, using buttons, not stylus, with a choice of only musical accompaniments unless more are unlocked later, but one of those is the Super Mario Bros tune, so I probably won't ever use the other two.
The only "extra" game I've tried is table tennis, and it was great. The table is divided into panels. Every time you bounce the ball on a panel on your opponent's side, the value of that panel goes up by one. Win the "point" and you get the value of the last panel you hit.
There's also Champion's Road and what appears to be a game based on the 3DS's step counter for the Marathon.
So far I haven't been able to choose the character I wanted every time. After I picked an event, I got the choice of about 6 characters. Also, in single-player mode, the one on one events have been the final. Even when I lost, I was still awarded the silver medal. Hopefully other modes have more in-depth competitions.