Smoke headlights and foglights - Costs a bit of paint
T-Cut rear badges - Free if you've got T-Cut lying around. Can also use fine sandpaper
Clean Clean Clean.. Clean arches, alloys, engine bay, clean everything! Makes a big difference.
Boot builds.. Can cost a fair bit for the subs and amps.. but the build itself is just the cost of the materials used. I did a twin 12inch boot build for 20 quid. Mind you the sub and amps cost be about 200... But if you've already got that, or you can get it second hand for cheap!
Lots of stuff involving fiberglass... iPod holder, Speaker pods etc etc.. be creative!
Lowering the torsion bar.. Free
Dewiper - Free!
Unblock the rain pipe on the sunroof so you don't get dripped on every time it rains!
Paint brake calipers..
Change the LED's in your dials.. - Few quid for the LED's.. and be VERY careful!
Refurb your alloys.. - Bit of paint and a lot of time. Maybe 30 quid..
Clean your throttle body.. Free, and helps throttle response!
Smooth boot lock..-bit of fiberglass, some paint, and a boot popper. Maybe 30 quid max