From a fitness and exercising point of view, Activity Log is actually very interesting. I've always advised my gym clients to use Wii's calendar to keep track of their exercise gaming progress, but now they can do the same on the 3DS.
While the Wii's calendar was limited to tracking the different games you were playing, the 3DS also tracks your walking around movement each day. The motion sensing part of the handheld means that you can pop it in your pocket when you go out and it will track your steps.
The Daily Records part of the software then provides a graph of how active you have been through the day. As I found when using Personal Trainer Walking on DS ($9.99 on Amazon) with my clients, having an immediate visual feedback of progress is a great motivation tool.
While Activity Log doesn't offer the in-depth analysis of your movement that Personal Trainer Walking offers (mapping periods of inactivity and activity to different times of day), the principle is the same. All the clients I have tried it out with have come back for their next gym session surprised at the results. It seems that we all imagine we are much more active that we actually are, but all it takes is some hard exercise evidence to set us straight.
Battery life is a slight problem here as I can't prescribe a client a full day's step tracking with the 3DS – they will need to recharge it after a few hours. The handheld is also quite large (although no bigger than a DSi) to carry around while exercising. This is one of the benefits of Personal Trainer Walking having a separate pedometer that didn't need as much juice.
After some tests it seems that with the Wireless comms turned off and the console in sleep mode you can track your steps for a day providing you don't do too much gaming. Not ideal – but these sorts of compromises always exist when you are using games to help your fitness and exercise plans.