I've already done all of these:
1.) Inverter as you mentioned for cell phone chargers, etc... ($30 Walmart)
2.) cathodes for interior lighting ($10 for cathodes / $3 for wire extensions)
3.) convenient or in-dash locations for ipod/cd player/flash drives for alternate data ($6 for bondo, dremel you typically already have, paint = $2)
4.) sound-sensitive lighting/strobes for parking lot fun

($10 for lights / $3 for wire extensions)
5.) convenient place to plug in your buddy's xbox/ps2/etc
(inverter / extensions / RCA+VGA extensions...)
any type of interior lights always impresses.