Hi mate.
Can't think of any reason why tints will affect your manufacturers warranty.
Just on the visual side of things you could also remove the rear badges and change the rad grill if you want to de
badge, fit a bee sting aerial, aero wipers or side air deflectors.
If you fit other mods such as HIDs, or a green cotton filter they can be removed and the original equipment refitted before it goes in for warranty work.
If you don't already have them you can also fit a glove box and/or boot light which can be left in place as they are original Seat parts.
Changing the sound system is no
problem either as any new equipment fitted will be covered by the guarantee from the company who fit it.
As mentioned by Elfman, you should tell your insurance company about any mods.
For what it's worth my car has been in for warranty work 3 times and it's got a remap, HIDs, 20mm rear spacers and a badgeless grill. None of the mods have ever been mentioned!
Hope this gives you a few ideas.
Regards Guy