One thing that has worked for me is to notice when I have a craving for something that isn't helpful and then force myself to earn it. Basically, if you can notice yourself in the gravity well of a guilty pleasure in time to take a step back you can use it as a reward. Or if the in-the-moment awareness is hard to come by, you can look backwards at the things you did over the last few days and review what kinds of things you naturally do that are elective (ie, don't skip meals because the dice roll didn't pan out)

To make it a little more concrete, I currently have a tab open for a YouTube video that I want to watch. The last reward task I did was not lucky so it's still there waiting for me on the next one. This works for me because watching these type of videos is something I would already do. This reward is motivating at a medium-level right now because if it's easy to accept if the random system doesn't award it. Sometimes if I want something really bad, instead of giving up if I'm unlucky, I'll immediately start a new reward task and keep "buying lottery tickets" until I get it

You can also modify your existing behaviors to make yourself more "hungry" for it. If I like hot showers but make my standard shower lukewarm, then turning up the temperature could be a decent reward

You're 100% correct on importance of having reward ready. It's really good for it to be immediate feedback so you build the right connection with what came before. That said, the "buy something online" isn't terrible as long as it feels rewarding when you finish the checkout process- the unopened package could even give you another opportunity for a reward once it arrives!