I'm looking for a way to boost my slayer level without doing the actual slayer tasks. I am aware that Troll Invasion and Penguin Hide/Seek rewards experience that I can place in slayer. Is there any other minigames I can do to earn a boon of exp?
You can do:
God Statues - Build 4 statues every month devoted to a god. If the statue you build is not the God associated with the area, a fanatic will appear. You can kill him for some nice slayer experience.
Soul Wars - This isn't really recommended, but it is an option. If you are on a winning team, you get 3 zeal. Tie and you get 2, lose and you get one. Zeal can be used to purchase experience including Slayer. The amount you get depends on your level.
Keep doing daily spins - A nice way to get daily experience is through the Squeal of Fortune. Continue doing this daily and use your lamps on Slayer.
During September, do the Super Challenges - Every day in September there is a new "super" task being released. After completing one, you get experience in the skill of your choice.
If Slayer is your lowest level, you can do Tears of Guthix every week.
Wilderness Warbands - You can only do these three times a day (they spawn every 7 hours) and there's also a chance you won't receive any experience. Killing any minions at tents will reward you with slayer experience. This is only recommended if you are high levelled.
Jack of Trades - An aura that costs 15,000 loyalty points. When you activate, you have three hours to gain experience in ten levels. After doing so, you can talk to Xuan for a reward book, rewarding experience in any skill.