Sign up to betting site Coral and when you deposit and wager £10, you’ll get £34.50 cashback from TopCashback. Thats at least £24.50 profit, or even more if your qualifying bet wins!
You could also look into ‘Matched betting’. (See Profit Accumulator NoBrainer for a further £5). Although slightly complicated, you could use some clever maths trickery to make your initial bet almost risk free. Do remember that this one is about free cashback though, and not silly gambles!