IMO, before you worry about aftermarket performance or big appearance mods, consider:
check your oil at every half-tank of fuel, to get a sense of the rate of oil consumption. If you've done that 4-6 times and the rate isn't bad, check it at least every tank of fuel. (Rationale: these engines are known to consume oil, but it depends on a number of factors; neglect leads to low oil level and engine failure.)
if you don't know the coolant history, consider doing a flush and re-fill. (Rationale: if it's never been done before, or more than 5 years ago, you can flush breakdown contaminants and restore some corrosion protection to the system with fresh coolant.)
if you don't know whether the spark plugs have been changed by that mileage, you might examine or replace them. (Rationale: affects combustion quality and electrodes are likely degraded by 150k without a change. NOTE: if you've never replaced plugs, watch plenty of videos, and consider getting an appropriate torque wrench -- and find an electronic copy of the factory service manual, if you don't already have it.)
Check out whether your stabilizer ("sway bar") end links (front and back) or outer tie rod ends have play; replace if so. (The whole suspension deserves assessment, really.) Double-check the passenger motor mount (by the coolant reservoir) hasn't failed (it's a common failure). Have a look at the trailing arm bushings to see if they're blown (see YouTube if you aren't familiar with them). Verify that your CV boots or ball joint boots aren't torn/leaking.
if you have only 2 keys, it'd be a great time to add 1 or 2 more for safe keeping. When you have 2 unique programmed keys, you can self-add up to 9 total. If you only have 1 programmed key, you'll need the dealer to add another (or have a locksmith clone the sole key...).
If the brake fluid has never been flushed, or it's darkened, consider having it flushed (or learn how to do it yourself).
Your paint and headlights look super clean. Research what protective coatings are recommended to keep them looking great for as long as possible.
These recommendations won't win you any superficial popularity contests, but they'll shore-up the foundation of your car, so that any performance or appearance mods you eventually get into are less likely to be tarnished by one failure or another. FWIW, here's an album of things I've done with my 2006:
After you do all of that to your car that hatch06 said, an OEM big brake upgrade is easy and really makes a difference. You use the rear rotors, pads, and calipers from a Mazda5, and the front rotors, pads, and calipers from a Mazdaspeed3.
Everything is direct bolt in, and there's a guy on here that got his sedan to do a 60-0 in something like 89 feet! That's friggin Ferrari level braking!
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