How can I get more money without paying for it?

Play the puzzles. The first time you complete a level, you'll be given a coin for it. The second time, you'll earn 2 coins, and 3 coins for the third time. To make sure you get to play as much as you want, you may want to purchase the Unlimited Lives for 6 hours which will set you back 69 coins. This might sound much, but during that period (if you managed to play continuously), you'll have no problems earning that coins back. You might want to spin the roulette every 24 hours too but there's just a slim chance towards earning the coins there (but free nonetheless).