As far as I know, the Nokia N800 (and, perhaps to a lesser extent, the Nokia 770) are always trying to use as little power as possible while still being usable. This is a feature of the CPU itself - if CPU cycles are not being used, then little power is being used. Certain software may run in the background, though, so be careful about what applications you choose to install and run (especially with applets).

Aside from the CPU, then, there is the screen (whose power-draining state is visibly obvious), the memory card interfaces (which should only use power when files are being accessed), and the radios. The radios can be disabled entirely via "Offline" mode, but otherwise they will occasionally search for access points (unless you disable that as described by aflegg). If you're not in Offline mode, then applications can trigger an auto-connect if they try to access the internet, so, again, be careful which software you choose to run in the background.

So, other than being careful about your software, you shouldn't have to do anything special to minimize power consumption - maemo does all that for you.

As for cron jobs: I think that those will still run in any mode other than completely switched off. Alarms will work. Audio will work. Pretty much everything works in every mode except for the radios (which only don't work in Offline mode) and the screen.