I've thought about this myself, but really for an "in between deals" stage. Ideally I'd be investing in real estate, but with down payments, the reality is there is a considerable amount of time to build up the next down payment and you want your capital to be doing something. Higher velocity of money means you're getting richer generally.
We did the "open a new bank account thing". Seemed like a lot of hassle to us, needed to have $15k in the bank for 6 months straight and 6 months of a direct deposit going into it plus X number of transactions coming out. All for $250 and it didn't seem worth it. Also, make sure YOU are receiving the offer. My wife's co-worker received another bank's Direct Mail piece, told my wife, she went to the bank and the teller said, "sure we can do that for you" but come to find out 2 weeks later, we weren't qualified for it and couldn't claim it.
Me personally, if this is really just play money, I'd explore Peer-to-Peer lending. Higher yields there and I've read it's fairly safe.