I'm sure you know then that a few weeks still isn't a lot of time to put something together, even if you do have experience. You'll probably want to stick to a simpler outfit then, like something that resembles street clothes, like school uniforms.
The next question is, what do you like? Video games? Anime? American comics? Give us some genres/series you're into and that will help us help you.
If you like Miraculous Ladybug, Adrien's normal clothing (blonde dude on the left
) could be a good option. Nearly all of the characters have a street clothes option, which gives you a huge variety of choices.
Pokemon trainers would be really apt for this summer, given the Pokemon Go hype. You could also do some simpler gijinka versions of Pokemon themselves, which gives you a bunch of freedom/creativity to do what you want.
Adventure Time's outfits are relatively simple sewing projects, and the wigs aren't too bad either, though you could have a ton of fun with props.
One Punch Man is also super popular right now, and Saitama's hero outfit is relatively simple, though it could be a difficult to work with material. There are tons of bald cap tutorials out there as well.
Best of luck!