What about some knitted socks from his mom? Or something silly or innocent. When I was searching Google for Superman socks, I saw some Kermit the Frog socks ("It's not easy being green" - I don't know, I could see Clark being a closet Kermit fan). But knowing how careful Clark is with his uniform, this would include everything down to his socks and underwear he would wear as Superman. That's why I agreed with Lynn that they were probably yellow, as there is so little yellow in his uniform. I went back and checked to see if we could see Clark's socks when he did the whole 'trying on boots' scene during the costume montage in the Pilot, but I couldn't see anything but boot.
And, yes, I was kidding about the striped socks earlier.
