There are loads of sat nav apps that store their maps on the G1's SD card so the phone won't use the Internet. I don't think there are any free ones though.
Paid for ones are reviewed on this navigation apps
page. Actually, also relevant to this thread is this page about G1 data roaming
. Make sure you don't have any monster phone bills by renaming your access point name before you go abroad. Then use airplane mode wifi to still be able to access wi-fi.