Hi there, I was in the same boat when I had my old 318iS. To be honest there isn't a huge amount of stuff you can do that will make a big difference without spending big bucks. If I was in your shoes though I'd maybe get a cold air intake and nice cat-back to (at the very least) improve throttle response and sound better, I would also remove any unnecessary weight, and fit the smallest wheels I would be happy with wrapped in the best tyres I could afford. That should make a nice difference

Edit: a good service would work wonders too, inc fuel filter and check for any small cracks in intake hose or anything like that. When you say sluggish do you mean more so than normal or just in general? I'd give the car a good going over and make sure it doesn't need any maintenance before doing any mods best of luck with it!