Let's say you have $100 in sportsbook freeplay. It's worth a little less than $50 EV.

Bet all combos on a three team parlay. Bet $12.50 an each of the 8 combos. A three teamer pays 6-1.

You have guaranteed 6*12.50 = $75 return on your freeplay.

Perhaps someone could suggest an alternative play with a higher guaranteed return.