This is a sample list of items and equipment you might want to take on a week-long trip to a beach resort in a developed tropical area, or some other part of the world (eg, Long Island or Southern California) during the local summer months. Depending on where you're going and the time of year, some of the items may not be useful to you, and there are inevitably additional items you'll want to bring along, but this should give you a general idea of what you'll want to pack.
Wear
Dry activities
hat, cap, or sunvisor
sweatshirt
windbreaker
poncho
2 t-shirts
1 nice shirt
2 pair of walk shorts
1 pair of long pants (suitable weight for current weather)
2 pair of underwear or racing trunks or bikini bottoms
ladies: 2 good sports bras or bikini tops
2 pair of crew socks
sneakers
Wet activities
2 pair boardshorts
spring or shortjohn wetsuit (May want heavier grade wetsuit, or no wetsuit, depending on conditions/activities)
rash guard
reef walkers
flip flops
gentlemen
speedo (Helps getting into wetsuit, and good to have on if the boardshorts come off)