I have a 1999 C280 Sport and would like to increase the performance. What are my options ?

You have a lot of options, but it depends on how much you are willing to spend, and also it depends on if you want a moderate bump in power, or a huge bump in power. If you can spent some money go for the tuners, Lorinser, Brabus, or RennTech. If not, just try a chip, exaust and other small mods.

Brabus are the benchmark in UK, I have reservations about
chipping,many firms do it but there are the Good,Bad & Ugly.
Tigerfox.

I would love to bump the performance a little on my C280 too. But when it's just me in the car, I sometimes forget just how quick it is. It really is a quick and peppy little car. I would be willing to spend an extra couple hundred bucks to get 10+ horsepower out of the car aswell.

__________________
2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)

Supersprint makes headers for the C280, as well as a piece that goes in place of a catalytiv converter you can remove if you know a shop that will do it and if you don't have to pass smog tests. Then get a high performance exhaust system and make sure you remove the resonator before the muffler section. HKS also makes an intake system called "Klasse" for the C280. This should all give you some hp gains, but removing the resonator along w/ a perfomance exhaust will make the car somewhat loud. These are all cheap to do, except the headers which are $1420 USD.

Yeah, I have thought about exhausts, but I am trying to avoid that. I do not want it loud, have a low note, or sound remotely like a rice rocket. I don't want to hear of feel my engine unless I'm really going! That's part of the reason I bought a Mercedes!
But thanks for the info... I guess doing something with the exhaust will still be a (maybe...) possibility sometime down the road.

__________________
2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)

Very good but expensive .....Renntech.

http://www.renntechmercedes.com

They only offer one upgrade for the C280 - cam.

c.f.