If you want to grind in the free version, level 2-6 (fabric box, #6) is probably the best spot. Cut the rope and pop 4 bubbles is pretty quick. You could probably get more than 4 bubbles per, by using the air bags, but I think it would take more time than a simple cut, pop, pop. pop, pop.
Since the candy drops, the puzzle restarts automatically.
PS. Took me less than 10 minutes (5 actually) after finishing the demo game and grinding the other (rope cutting) achievements.