SO what exactly you want? If I understand you correctly, you want your exhaust a little bit louder but nothing crazy right? The cheapest way is your muffler delete. Get yourself some exhaust tips and have a reputable muffler shop to do the work. You can wait at the shop while they are working on it and then just sneak your stock exhaust in your room until you wanna put it back on. You do have to make sure that your dad won't find out that all of a sudden your BMW is making louder noise and take a peak underneath your car.
Other then that, I would recommend RE Ikon exhaust (that's what I would do on mine) if you wanna go a lil baller.
How sensitive is your dad to car noises? My mom is such a pro when she comes across the mods on my car, she can tell if I have lowered my car or when I changed out the exhaust. However, she is not sensitive to cosmetic changes since she has not realized that I am on my second set of wheels already.... weird.