I'd say mainly shorts and tshirts, but be prepared for any kind of weather! Pack at least one pair of long pants, a sweatshirt, and some sneakers and warm socks!
Here's what happened to me. I was at WDW for 11 days last October, and the weather went from record-hot (90+ for several days) to record-cold (50s and very windy). I had only looked at the forecast for the first week, so I packed almost all shorts, tank tops, and sandals. I had one pair of long pants, one sweatshirt, and no warm socks. I was so freezing one night at Epcot that I went on a frantic search for warm socks (my legs were freezing, even with long pants, because it was so windy). I finally found one pair of socks at the Japan pavilion - calf-length socks that cost $12!! But I was so happy to find them, I didn't even care that they were meant to be worn with Japanese-style shoes (the kind that are like flip flops, with a divider between your toes!). Never wore those socks again, but was so glad to have them at the time!
So lesson learned. From this point forward, I'll always bring a couple of warm layers, including knee socks, sneakers, long pants, a warm jacket, and even gloves. It's hard to find that stuff when you need it, and if you're lucky enough to find it, it will be expensive!!