It really depends on the details.
Let's say they give you $500 to play with for like 1 hour. And any winnings, you get to keep? Let's also assume that there is a "consolation bonus" that you get if you lose some of this play money.
If that's the case, just play it big, as big as you possibly can. Find the game that allows the largest bets (they're probably capped at very small levels). Perhaps roulette, do it by scattering bets on roughtly every single spot in a one-third area. Your goal by playing big is to either quickly get that bonus amount up to a high level (you decide where) or run out of play money quickly, and get the consolation bonus.
Aterwards, the bonus is probably transferred to your real money account, but locked up until you complete a wagering requirement. Two options are to a) find the lowest-house edge game you can grind it out with, and grind it out, or 2) Bet big again, letting the bonus either multiply your winnings, or subsidize your losses (you'd want to at least try to double-up). This option is not for the faint of heart, because it results in losing your entire deposit at least half the time.