The old family trip to Reno when I was 9 and both sides of the family, both from my mom's side and my dad's side. Just hanging out with the cousins, playing in the snow, having snowball fights, singing stupid kiddy limericks about the batmobile losing it wheel or something, watching an awesome godzilla movie (Big G Versus Ebriah), then top it all off with the Circus Circus Arcade room. I will never forget any of those moments.
Unfortunately, when I got older, I realized everyone pretty much shafted my parents into paying for everything, there was alot of bickering with the older folks, and started a feud between my mom side and my dad side about favoritism. Makes me feel very shitty that I didn't realize it while we were there...
But... The child inside of me will never ever forget all the stupid kiddy friendship bonding time there... Hell, I think it's one of the reason why we are still so tight today. No matter what light was shed upon that vacation and how badly it messed up the family ties, to me it will always be a Cherished Childhood Memory.