[movie voice] How do I stop these... DEMONS?! [/movie voice]

My N900 will run fine for 30 hours on a single charge, with light web browsing and phone calls. I think you're fine.

For the GPS, the N900 wants to have an internet connection. So you're not going to be able to disable the cellular modem. But this means that your Internet-connecting apps will be waking up and doing data transfers, e.g., modest, apt-get, etc.

You can also kill browserd.