you could get some performance springs and shocks, and then just pull em back out when the lease is up. There are all kinds of computer chips out there that will raise your hp, torque, take off top speed limiter, and raise your rev limiter, that cost about $250. springs and shocks will run you anywhere from 700 dollars and up, depending the type you buy. Exhausts can be cheap(50-100) that will make it sound better, but if you want one that will kick some performance into your car, you might have to put more money into it. K&N cone air filters can be purchased for about 30 dollars, but you need a conversion kit that costs around 40 dollars. The cone intakes don't really do that much, only giving you about 5-10 horsepower, although sounding alot better.
Just be sure to keep all the stock stuff that you switch and put it back on when the lease is up!! But i wouldnt recomend switching big stuff like springs, shocks, etc. if you dont have the car for much longer.