A new or used CD player?
Cheap easy mods, speaker upgrades can be fairly straightforward, or for a minimal charge i'll fit 'em.
Other than that, styling or performance mods? Oh and bee sting whip antenna i can do at £4 in several colours. for LED lighting i'm the man too.
Performance theres a little trick with spark plugs.
If you mark the body on the side the gap is at when you remove them, so when you put them back in you can tell if the spark faces the injectors when you put them back in you'll get a little better performance/fuel economy. Plus its free!