Having completed the campaign, I only play Plants vs Zombies intermittently now. However, the completist in me still wants to get the gold trophy and all of Crazy Dave's stuff. To do that, I'll need to earn in-game cash. I've found the Last Stand mini-game to be ideal for this purpose.
In case you haven't reached this level yet, Last Stand starts you off with 5000 sunshine. You can plant as many seeds as you want without waiting during the setup phase. Caveat: sunflowers may not be selected. While 5000 sunshine seems like a lot, it's actually not. The configuration above shows my optimal solution. I can leave the scenario running without having to attend to it as the golden magnet automatically captures coins and diamonds. I add the coin-generating daisies after the first flag.
This configuration generates $3,000 to $4,000 in-game cash for the entire mini-game.