I don't think that there's a lot that you can do with the SR18DE that you have. I mean, there isn't much more for the SR20DE, but the thing is I personally think that a 1.8L engine isn't made for any sort of aftermarket stuff.
With that said, if you want to or are kind of stuck with that platform (I'm stuck with my SR20DE which is great in a sense), then the best that I can tell you is to get the small aftermarket stuff. For example, I'd get a 2.5" Cold-air intake from K&N or something and then get the ECU flashed. Well, maybe don't flash the ECU. I'm just thinking that there should be a way for you to take off the rev limiter on your engine which will allow you to have more control over it.
In terms of other stuff, I'd suggest looking for mufflers. Greddy and Tanabe are my favorite two to look at. In terms of a nice aftermarket look and not all out performance, go Tanabe. Tein suspension systems might work if you have an '01-'06 platform. I'm sure there's a bit more than what I've stated, but your best bet is to just wait 'till the engine dies and then get a SR20DE, SR20VE, or the SE-R engine (QR25DET?). In that way you're not restricted as much.
Hope that helps.