So far, I've only done some aesthetic mods.
I got a set of LED DRL headlights, but they weren't cheap at almost a grand. I love the way they look and work though. I had to bridge the marker light fuses to the seat heater one so they are on as DRL, since the car doesn't have the harness for doing them properly.
I also got a set of smoke LED taillights and a smoke LED third brake light. Those two aren't nearly as costly as the headlights, and they look awesome.
My car is a 2001 base, but it has S wheels on it, which also look much better. They were on it when I bought it. What I did to them was plastidip them in black. They look great, and one of these days I may fork out the dinero to get them powder coated. So far, the Plastidip has been on them for about two years with no issues.
I've also bought a small wing spoiler to replace the stock one. It's one of those small ones that will still raise and lower just as the stock one does, but when down, it has a lip more like the newer models. I haven't gotten around to installing it yet. It cost a little over $100 on ebay. It's the same as the one they have on the yellow car in the youtube video titled "Porsche Boxster Hamann Body Kit Install."
I'm looking at doing some performance mods next. I'm thinking de-snorkel and Catback exhaust. I don't mind a little noise.