So what random acts of mayhem and fun do all of you do in open world games?

GTA IV- The things I remember most was this one. I ran into a hospital/small clinic and held it up. I shot numerous people inside and let some go to attract the cops. There was one entrance so I barricaded myself behind some desk and let the cops come in the automatic doors one by one until I shot them. I would go for as long as I could until the whole swat team eventually breaches the clinic to mow me down in a flurry of gunfire.

Read Dead Redemption-I would use my lasso to tie people up and drag them behind my horse. It was pretty funny to tie them to the train tracks too.Some more classic stuff like shooting up a saloon just for the hell of it.

Far Cry 3- I would use my anti material rifle and wait on top on the hill. When cars pass by I would shoot them and blow them up.

Assassins Creed 3- Just go around town hanging people and killing them really.

I let a Nopon child drown in a lake.

In Xenoblade I try to find the highest point where I can jump and survive - there's a spot in Valak mountain where you can jump into a hot spring from the peak and survive. The eryth sea also has some incredibly high points you can jump from.

And I wouldn't describe it as 'open world' but in Last Story I still love pranking npc's with bananas shot from my crossbow - the exaggerated way they fall is hilarious.

I just like driving as fast as I can in GTA and see how far I can make it before I crash.

I help the elderly cross the street and pick up empty cans and throw them in the trash.

Ok, not really. I usually end up stacking cars and trucks behind each other on subway, train and tram tracks.
Driving a big passenger plane on the road was fun too in JC2.