If Sumperman decided to use a utility belt.

What do you think he would keep inside it?

I personally think he would have an assortment of gadgets that include, basic magic charms, a way to teleport(himself and enemies), and a general blocker of radiation.

LOL Red Sun sunblock

I'd like to see Superman have a suit that replicates the purpose of this suit from All-Star Superman

and

a Motherbox

which would protect Superman from red sun, protect atomic manipulation, open BOOMtubes, heal Superman, the ability to change appearance (Think about the possibilites for Clark Kent), redirect energy beams, and much more.

The only thing I would add is having one of those miniature suns power the suit shown above so he couldn't be drained easily and could take into the extra-power if need be and have it be resistant to kryponite radiation

As for the utility belt Superman could just have a mini-BOOM tube to access the following things from a pocket dimension or something:

That gravity gun he used on Luthor in All-Star Superman

The Phantom Zone Projector

The Key to the Fotress of Solitude

A device to access his Superman robots in case Superman needs back up and needs someone to stand in his place

Maybe a miniature Sun

Encouraging notes from his friends and family, because that's all he really needs.

The crazy thing about a belt is it is yesterdays news. With a suit that is tech based micro robots or nanobots he has anything at his command that the robots can make. If he wants it he simply have to think it and the small robots go to work making it a reality. So where as batman seams to have the upper hand Superman can say this is not longer the case. A supersuit that changes to the environment. Even going as far as to replicate to create a ship taking from space dust or creation micro worm holes into other realities and pulling forth what ever was needed to get the job done.

Truly a super suit if ever there was one.

Leave the belt for holding up pants.

The anti-batman-Spray!