Almost all of the balconies in the game have collision, and can be stood on. The best way is to speed at them with a helicopter and jump out at the last second. From there, you can utilize it as a sniper spot. Be careful, because some balconies have missing collision in some spots, so you may end up walking through a building or directly over the balcony ledge.
On Hematite Street on the east side of Algonquin, there are four tenement buildings (2 on each side of the street). In the corners of the buildings, you can find open squares that drop to bluehell, usually covered by trash.
On the service road that runs east of the airport, there is a spot where the curved pillar meets the airport wall. Jumping into it just right can let you hide in the airport, and you can shoot out at people while they can't shoot you!
In Ballad of Gay Tony (requires parachute equipped), get on top of one of the large tenement buildings in South Broker. They're the ones with the American flag hanging off the balconies in multiple spots. Each building has four wings and on each wing there is a slightly taller part on the outside edge with a shorter middle path that leads to the center of the building (if I remember correctly). Jump from the taller outside part to the center pathway part. Instead of just doing a simple hop down, your character will go into the skydiving animation and glide across the top of the building and the wind sound effect will play, until you hit the center part of the building and go into ragdoll mode. The best part about this is every time you do it, your health meter will go slightly up.
With enough persistence, you can jump into the china shop and use it as a hideout. When you're ready to escape, jump through the back wall and you'll fall and respawn outside of the store.
Cars drive fastest on the Plumber's Skyway and the highway that runs on along the western side of Algonquin. Try to get pedestrians to chase you into traffic and watch them get hit, or try to smack motorcyclists with a bat.
Outside of Lucky Winkles, there is an underground cellar that can be entered, complete with opening doors. This spot is useful for trapping people in multiplayer. Once they're inside of it, just drive a car over the top of it and leave them there.