You won't need a Passport. Photo ID will suffice.

My relatives are in the U.S. and they have come up here with all kinds of musical instruments, heck they even had an Amp, P.A., and electic guitars and Basses. (I host a gathering, Latimerpalooza that this gear was for). They have been asked about it and have just said that they were playing a family party and there has never been a problem.

My sister has had two guitars in airplane and when asked has said she is just playing with her cousins and it hasn't been an issue.

Me? I have even less to worry about. I know that if they ever pursue the fact that I have my Five String in the car, all I have to do is take it out and play it, they'll know that I'm not going into the U.S. to earn money from my playing.