probably landscaping. some of the work is really hard, but it's a damn good workout, and you'll pick up skills that are useful later in life. the rest of the work is dead easy. it also pays a lot of money, up to $15 an hour depending on the company. construction is also pretty fun, and again will teach you useful things. any manual labor is good for a summer job if you can stand it, because it's usually fun, teaches you useful things (ladies love a handy man,) and most importantly it pays a lot.