Yeah, get a racing battery, least than half the size of a normal 12volt.
Only issue is you wont be able to leave car without starting it as long as you could normally.
How about changing steering wheel to aftermarket, lose the airbag system.
Go for smaller lighter rims, Bucket seats? Ditch the roof lining?
Rear speakers as someone mentioned, will amaze you how little difference it makes audibly. GP stereo is almost as good as my S.
Failing that Go on a diet yourself? Greatly depends how drastic you want to go.