Another vote for the beach. Loved it as a kid of course, because I was free to run, dig, swim, have ice cream and chips and only carried my bucket and spade from the car to the beach.
Fast forward 30 odd years, now we argue about which beach, whether it's best to go 'high or low tide', parking charges, where to sit, who sits with the mounds of stuff and who takes the kids in the sea (too small to go by themselves), why we should eat the sandwiches we made instead of queueing for overpriced ice creams/chips/lollies/coffees/teas/candy floss.
Spend all day fussing about with sun cream, hats, swimmers on, then off, then on again, get too hot, or usually too much wind in your face, get slightly burnt, get back to the car to find an enormous long queue to leave the car park, drive home 2 hours, try and persuade kids to go to bed after they slept the last 2 hrs, fall into bed feeling grubby, salty, knackered, wake up next morning feeling hungover and face the pile of left over picnic, sandy swimwear, toys and crap all over the hallway, in the car, in the kitchen.
Go to work.