Help! how can i make money??!?

so heres the thing... i am a 15 year old girl. i do not have a workers permitt or have never worked before. but i really need some money! my boyfriends birthday is in a month. he is turning 16 and our 1 year anniversary is a week after his birthday. me and my family are going to be out of town on his birthday so i wanted to get him something really nice and something he really wanted! he said that he wanted a muffler for his car... i promised him that i will find a way to get it for him. the muffler is $130! my parents will probably not help pay for it. i really want to get him this present! he has even tried giving me money to help get started but i wont let him. i want to make this money on my own. any suggestions?! i also tolled my dad i would work for him (he works on ariplanes) washing airplanes or sweeiping up but he said that he has someone that does that! ugh! im stuck!! how can i make fast money!!!!!