I have only done one truly memorable spontaneous act: Participating in a prank over granola bars...
Around the end of senior year, I was hanging out with my group of friends during lunch, like every other day that year. "Friend 1", while Friend 2 went off to get a snack from one of the vending machines, decided to take one of 2s granola bars from his lunch bag. 2 didn't seem to notice, and we finished our day without 2 calling out 1 for his grave misdeed.
Now: 2 always brought lunch to school, because the school-served lunch was atrocious on a regular basis. He also had a penchant for pulling minor pranks on people that upset him. These granola bars were his crack, and he did not enjoy losing any of them. This was a call to war for him.
2 calls me later that day, and tells me that he knew that 1 took his chocolate granola bars, but just played dumb to prepare for a prank. He tells me to meet him at 1s house the next day, and bring clear tape.
When another friend and I arrive, 2 led us into 1s house, and tells us to start taping up some posters he brought all over the walls.
Each poster had either a creepy/demonic image and related text, or a granola bar on it. The granola bar posters (about 20) were each placed in 1s room, and all said "WHERE'S MY GRANOLA BAR 1!?!? I KNOW YOU TOOK IT!!!!!". One was placed on his pillow that had an image of the devil from Legend on it, and told him to "GO TO BED WITH YOUR LOVER!!!". Each poster was creepier and more rambling/insane than the last.
We drove out of there just as 1 was supposed to be heading back to his house. About 5 minutes into the drive, 2 and I get a call from 1. He's freaked out, and asks if any of the guys had been in his house lately, to which we decline.
At this point, he starts to panic, and asks if any of us are nearby to check on him, asking about criminals in the news, etc. It's at this point that he pauses in his freaking out, and asks 2 if he knows anything about granola bars. 2 and I lose it, and give 1 a speech about not stealing a persons granola bars. 1 said that he was in his kitchen with a penknife, and was convinced that a killer was on the loose or something.
All was forgiven later. It was awesome.