I've been carrying my 9C for over two years and I love it. The only change I made was an upgrade to night-sights and I sold the CTC laser because it made the gun not fit my hand. I'd consider a compact rail-mounted laser or light but other projects have taken priority and likely will for a while.
Is your 9C going to be for carry, home defense, fun at the range, etc?
If you're going to use it for home defense you might consider a weapon-mounted light such as the Streamlight TLR-3, TLR-4 (with laser), or one of the Viridian light/laser systems. There are plusses and minuses to mounted lights and lasers, but with training they are both very beneficial tools for the tool box (Have a hand-held light as well).
If you plan to carry the pistol, I'd recommend, depending on your dress, lifestyle, etc., picking up a full-size 17rd M&P magazine, possibly with an X-Grip spacer, and use that for your reload. Having 30rds on board is a nice feeling. I use the finger-rest magazine in the gun and the 17rd mag as a reload. When I was carrying the 9C as an on-duty backup I used the flush-fit magazine. Other than that I find the extended finger-rest magazine doesn't really result in any more difficulty in concealment but it is much more comfortable and solid to shoot.
If it's for fun at the range, look into the Apex trigger/action upgrades. They really turn the trigger into something nice. For self defense, you would want to keep the trigger at a reasonable weight, but for the range, there's nothing wrong with making the gun more "shootable".
Lastly, if you want to start personalizing the gun without permanant damage, order some extra backstraps in your preferred size and break out the soldering gun for some stippling. The backstraps are only a couple bucks so if you screw up it's no big deal...It's a nice, cheap, and harmless way to personalize your pistol.