I've been having fun with some minor mods to my '97 855 NA over the past 5 yrs.
Started with Mobil 1 and the engine had 110K on it and varnish on the dipstick. The varnish is gone now and I have to think that things a cleaner in there now. I've also read about sludge killing cars and feel better protected. I change around 5K, but forgot once and went to 9K, you can't do that with regular oil.
I don't worry much about the trans fluid. Had it done.
The therm is not a big deal. It's for road temp and is not accurate for outside air temp. It tells you when it's 32 and freezing but you usually already know. The other 'tronics also don't seem worth it to open up the cluster. You also might have to turn around and open it again if you push the trip counter and break the little gear in there. (I haven't)
I like to get the 91 octane. No one has it except for Sunoco I get it when I can and don't sweat it the rest of the time.
Haven't done headlights, The stock are pretty bright and I don't want people flashing me on the highway. I did do side marker lights from IPD though, and like them very much. I ran the wiring and it wasn't hard to do.
Other things I got: Wiper relay from IPD, K&N air filter, cabin filter adapter, Side marker lights, added V70 Euro rails, added v70 spoiler, changed out steel wheels to Volvo Columba, IPD exhaust tip, Bilstein shocks, just got Zimmerman cross drilled rotors, I want to add the 3rd seat, get the IPD skid plate, perhaps get a hard wired iPod adapter, and I've just noticed that you can change out the air dam to one from an R model (Volvospeed.com).
There's always something fun to look forward to and this site is great.
Have Fun,