N900 still can:
CLI for SSH, Maebble for managing Pebble smart watch, OMC music over FM transmitter(for older rental cars), sony MH750 (using OMTP->CITA adapter) or mh755 with collar clip Jumbl BSR36 bluetooth, or bluetooth speaker tube; music managed with fwd/rev/pause/vol from Pebble watch, telephony, SMS(also bridged to watch), MMS, audio books.
HeySMS to bridge SMS via wifi/Pidgin to my laptop is still huge and without a replacement.

No longer:
No more browsing, email is pretty hosed, gps nav is wonky, rtl-sdr was always a niche use with the N900, finding web radio is not something I worry about, FM rx was never a big deal, I am not playing the old Palm Pre games though they are still as good as anything on android, I am just done with mobile games I think,

Tablet bridge:
If I need I bring either a Nexus7(flo) or a cellular-neutered S4mini (software disabled and disconnected antenna) running a cleaned up LineageOS with microG for web browsing, nav, Kodi streaming, and other mobile computer stuff like managing my NFC transit card. I bluetooth tether these devices to my N900 for internet access.

Future:
I am waiting for my Pinephone and then this N900 will either join my other 8-10 broken ones or go to a kid; that is once the Pine is ready to replace ALL of the N900 functions including heySMS and become my everyday phone. Until then I have a mirror SIM waiting for my Pine so they can both run on the same phone number. Hoping for all of the good stuff to work on Leste-4-Pine.