I would like more information about "bartering" as a way to earn and report income and thereby earn social security credits.

I'm retired from the federal government, from which I receive a monthly annuity.  I worked in the private sector a bit before I started my federal career so I do have almost enough credits to receive social security.  I'm interested in what the IRS calls "bartering" as a way to earn the remaining credits.  (I'm aware of the offset for those receiving a federal annuity, which reduces, but does not preclude, social security benefits.)

I volunteered to teach English in Spain in exchange for room and board earlier this year. I estimate the value of the room and board to be about $1300 - exactly the amount needed for one social security credit.  If I report the value as income to the IRS for this type of situation, will social security credits accrue?  

So the question arises - what sorts of barter experiences will earn social security credits after I report the value of their benefits (usually room and board)?  For example, I'm also interested in volunteering at various retreat centers.  They typically discount their fee in exchange for work.  So if the fee were $1000 and I volunteer for a period of time and they give me a 50% discount, would I report $500 as income (toward social security credits)?  

How about volunteering as an usher in exchange for seeing live theater?  Would the value of the theatrical production be taxable thus earning me social security credits?  Thanks for any input.