external GPS server
GPS → old phone (calculates position) → bluetooth → current phone
This relieves your current phone of the workload of tracking and calculating a fix, which costs energy. Bluetooth uses much less energy so your current phone only burns energy keeping the LCD lit. It would increase navigation range on a charge because effectively you would be using two batteries. Also avoiding the battery performance hit due to heat because the processing is distributed. The problem is I think no FOSS nav apps support external GPS. There are FOSS apps and drivers to feed and read the mock gps but the nav apps don’t use it.
bluetooth radio receiver:
Old phone has bluetooth enabled and pairs with whoever at the party wants to be the DJ. The headphone output goes to a channel on the (otherwise bluetooth-incapable) mixer or amp.
fake hotspot:
Setup a hotspot with no internet uplink. Use the SSID as a bumper sticker (e.g. “ImpeachTrump_optout_nomap!”). You could theoretically run a web server on the phone which redirects all access attempts to a captive portal that broadcasts whatever msg you want (e.g. anti-Trump memes or announcements for neighbors). It need not give WAN access.
Maybe incorporate Rumble: https://f-droid.org/en/packages/org.disrupted.rumble/
cryptocurrency:
It could serve as an offline/airgapped cryptocurrency wallet.
car telemetry:
Keep the old phone permanently in the car and attached to the OBD.