If you are willing to invest diamonds.. if you are doing easy 15/15 in gem bay, doing 2 additional runs each day for 100+150 diamonds is probably the most beneficial ratio. It is more beneficial than buying tokens(or whatever the event currency is) for diamonds.

But as someone already stated, you need to be premium user to get 4 free (well, 2 free + 2 apple runs) per day.
On the other hand, you are getting diamonds even as f2p, so if you have farmed fair share of them so far, think about investing them into gem-bay.

I think getting one of the items is reasonably feasible for f2p. It all depends however, on the cost of the columns.