For the past few evenings I've done the following to my 2006 Scott Sportster P4:
Stripped rear derailleur, cleaned, greased and reinstalled
Installed new cassette - I now have a shiney one!
Installed new chain
Cut bars down by 4-5cm and washed grips
Installed new pedals - old ones had loads of play, just went for light open pedals
Adjusted saddle and got rid of squeak
Swapped old sus forks for rigid alu Kinesis ones - had to cut down steerer, which I overdid by a few mm, almost destroyed crown race getting it off old forks, and well and truly mangled compression bolt fitting. It was almost a complete disaster, but I rescued it by hitting it with a hammer!
I also recently got Cont Gatorskins and new tubes, which I pump up to 120psi.
I bought the bike second hand for £150 earlier this year, all in all I've spent an additional £210+ - I know I know, I should have bought a new bike from the off. However its been a good learning experience. After riding BMX's exclusively for the last 15 years pretty much everything on this bike is new to me!
Bike feels like a new bike now though. I managed to get 45mph downhill on it when it was pretty much falling to pieces, looking forward to my commute tomorrow now - should be good for 50mph I reckon