Ice cream, cold water :) LOL Frozen Yogurt!
Have you thought about fans? In the summer there is a major demand to remain cool while battling the rising heat. Or Flip flops, knockoff sunglasses, towels, and cheap jewelry, sun creams, straw hats, sunglasses, shorts, skirts.
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My idea for selling something unique is already in the market in the name of Koozies, for enjoying the chill of beverages from opening the bottle till the last sip. The option to customize the print on it has made it a big hit for various parties and celebrations.