Too tiring to do the 2hits/blackjack. Hand cramps after 15minutes
You can learn to 1 click the summer garden. Very good xp, almost on par with pyramid plunder. The only problem is if you screw up or bank the squ'irks, it takes takes a while to recreate teh correct conditions for the 1 click.
I would say pyramid plunder is the fastest thieving exp in the game. You need to have started Icthlarin's Little Helper to play this activity and you receive up to 200k exp per hour at level 91 thieving or over.