this one would be super easy. just cast the stitches onto the circ instead of DPNs and go to it (knitting stockinette in the round is just knitting every round....no purling required!)
you don't have to change colors either unless you really want to.
i have a hard time getting most baby hats to fit on circs but someone may have a pattern that will work for you!
do you have the circular needle already? btw...you will likely need DPNs to finish the hat because as you decrease the number of stitches around will be too few to get around the needle. It is super easy to use them at this point!