No seams! (Okay, very few seams.) Hats on small circular needles go incredibly fast. When you knit on circulars in the round, it's always a good idea to use a stitch marker (even if it's the plastic ring off a soda bottle neck) to mark the beginning of rounds. When you change colors, it does leave a slight jog unless you do one of the "jogless join" techniques, but most of the time it isn't too bothersome.
For maximum lazy summer day knitting enjoyment, not to mention in-the-round practice, I recommend a 16" needle, a ball of self-striping worsted weight and a plain stockinette hat pattern that starts with the ribbing on the bottom. When you tie off the top, it's done and ready to wear. The only tricky part is learning what to do when you have too few stitches on the needle to keep going as you have been, but that takes about five minutes to figure out if no one is there to show you.