That's an oddly hard question. To be frank I'm generally happy or at worst content (or a bit bored). I can think of various times when (though it didn't twig so much at the time as I was too busy doing it) when I was particually happy.
Loosing myself in woods, finding an abandomed hospital in thick fog (was very creepy too), spending time with friends etc etc.
Though now I'm dragging up memories I can recall one of the favourite things we did as a kid.
For a number of years when I was around 10ish age bracket I think, we had a boat called Sarala. She was a Snapdragon 747, which is a twin keel, sleeps 6 style boat.
For sake of completeness here's a pic though it's not of ours, none of those are in electronic format I know of. In fact if you're really curious there's one for sale
here.
Me and Billy would sleep at the bow, pretty much under that hatch you can see. One of my favourite choices of destination on it was on the Isle of Wight (this is also the holidays where one of the best / worst jokes my dad ever told me and I repeat too often comes from "What's brown and comes steaming out of cows? The Isle of Wight ferry").
Details kind of went over my head (really should ask the folks at some point) however I mostly remember we'd sail it up from Portsmouth harbour where she was usually moored to this place. That it was a flat sandy beach on one side of this finger of land jutting into the sea where we'd moor up. If the tide was in you either used the tender or jumped in and swam, if it was out it was a case of climb down and walk. There was woods / trees up the centre and then on the other side it was solid rock / rocks covered with limpets which you could knock off, put in a net and watch fish just swim into the net. The size was limited by how big a fish you could lift up in the net without them falling out. I can remember days spent either catchign shrimp on the beech, climbing in the trees or rocks or fishing with nets. There were other people about but not many, there was a small town where this finger of land hit the mainland but I was always too busy to go exploring.
There was the addition of 'pirate ship' where I'd go swab the decks, or Swissfamily Robinson etc going on in my head half the time but generally it actually was quiet Swallows and Amazons except with my parents too.
I loved those holidays.