Fan grills, cathodes, blowholes, all good, cheap and easy mods. So is stealthing. I'm guessing you have adequate components or you'd be upgrading those instead of modding. Another good thing to do (not so much a mod as a tidying up thing) is to get rounded cables and use those, and zip-tie all your PSU cables. This will not only improve your airflow, but also will make your case look neater and your components more visible.
Or, you could take the whole thing apart and put it into a toaster
