FYI, My car is a Suzuka Blue '06. Here're some simple but sometimes very effective ideas to start with.

1. Modifry's RPM/Redline Shift buzzer - My 1991 RX-7 came with a shift buzzer stock, and I became addicted to it. I found Modifry's and installed, so now I have that on my S2000; very nice.
2. X-Brace - You can really feel the difference as less (undamped) bounciness over rough roads. I choose Rick's.
3. (Custom) Alignment - Set to your preferred specs. I prefer less rear camber for a street machine, so adjusted mine near the minimum spec (-1 is minimum, actually I used -0.9). The rear end feels much more stable, plus, much better tire wear. (Also feels better putting down lots of power during turns as both tires have good ground contact.)
4. (Custom) Valve Adjustment - Increase engine acceleration smoothness, increase throttle response crispness, increase full power especially 4-6k (e.g. below vtec), lose some power below 2k (doesn't matter though). Use minimum factory specs (0.008 intake & 0.010 exhaust). I had mine done at 4500 (4.5k) miles, and would do it again in a heart beat - really worth it.
5. Air Box Modification - increase throttle response (cut out internal divider, remove air flow directors), get better sound (block resonator port). Reported at 2-3 hp increase. Can't beat free hp.
6. Throttle body coolant bypass - decrease temps at throttle body (maybe don't do if worried about throttle icing due to operation in cold weather).
7. Coolant mixture - Improve engine performance in warm weather. Reduce/eliminate "kangaroo" effect on hot days. Drain some coolant and replace with (pure, distilled) water to acheive a 70/30 mixture or so. 70/30 is good down to about +4 deg F, I think. (Obviously don't do if you have a snowy winter.) Use Redline Water Wetter (NOTE: if you do this, especially if you use the WW, you must flush & change your coolant at least every 15k; I choose every 10k / once a year. It's worth it though.)
8. Experiment with fuel - 91 octane is the best we have here in CA, but still I have options. Here's a comparison of my available fuels. 91 Chevron: smooth, high driveablity, good on low end, but can't hardly tell when crossing into VTEC. 91 Shell: explodes into VTEC, there's no missing VTEC engagement!! somewhat less drivability in low end - a bit of occasional stumbling at less than 3k rpm (though coolant mixture and valve adjustment basically cure this). I choose the 91 Shell. (NOTE: both of these gasolines are TOP TIER.)