Outside all of my video gear that stuffs my luggage too much I bring:

2 pairs of board shorts
1 pair of pants (which I basically only wear on the plane and rarely when I'm just sick of shorts)
2 tank tops
2 or 3 T shirts
cheap water shoes
flip flops
1 pair of sneakers

And that's about it. As you can see, I can fit my stuff in a school backpack without much problem.