The first thing to say is that anything goes! Well, almost! Given that you've chosen to have a humanist ceremony, religious music (including a hymn) isn't suitable, but there are so many other options - classical, jazz, pop, rock, folk, you name it.......
You need music in 3 main parts of the ceremony - for the entry of the bride ('processional' music), during the signing of the Marriage Schedule and for the bride and groom to leave ('recessional' music). Think carefully about the mood you want to create during each part of the ceremony - for example, most people choose something light and reflective to accompany the signing of the Marriage Schedule (about 5 minutes of music is ideal, so 2 tracks is perfect), but then have something with more impact for the end of the ceremony - to make everyone smile and to get the party started!
I've had a few couple recently where the bride has chosen the song she wants to enter to and kept it a secret from the groom (hoping to get a tear out of him of course!) It's a great idea, especially if you girls are happy to let the guys choose the last track for your grand exit and keep that a secret from you! LOL!