Unofficial visit or official visit to me they are the same...and like the old quote says you never get a SECOND chance to make a good impression.....
That being said, you do not want to wear a uni or a shirt and tie, but just go looking like a good clean cut athletic baseball player.....
Polo shirt and khaki shorts or pants will be fine.....looking like the ad from american eagle or hollister will do fine.....
But there are things not to wear.......do not wear piercings....earrings, shirts with questionable imprinting (sayings) or another colleges team apparel....
Get a haircut.......(makes me sound like my dad) but its still true.....SHAVE!!........if you wear a hat wear it right.......
You are selling yourself!, understand that and youve got a head start......
oh yes, and one other thing....you can only control the impressions you make....so if possible dont take the other boys with you....