[dlsa]

You could try consequences. Some variation of the following might work.

Your diet fallback position now consists of chicken and broccoli and one meal per day. No sauces no famcy drinks, no coffee. You must walk the equivalent of 15000 steps. That is your "little hell". You come up with a different scenario for "little heaven".

There's your fallback position when you don't deliver on daily or weekly goal. If you do deliver you know what happens as well.

Now do your thing. Set goals etc.

The actual choice of food etc isn’t important. Hell: Make it spartan / Make it limited. Heaven: make it fun / expansive. Don't even need to make it food/exercise. Plenty of other options. Use your imagination.

[admissionsguy]

Your “hell” scenario sounds more like a baseline healthy lifestyle than a punishment.

[dlsa]

See? It sells itself.

Most people would see it as severely limited because they'd want to add some sort of poison like alcohol or ice cream etc.