What reward idea seems good?

Great idea. I think 'positive' incentives are better at motivating than negative ones.

I would try and keep 'rewards' healthy, edifying and personal. What do you like to do? Is an afternoon spent walking in nature with your phone left at home a treat to you, or would time spent with friends be a stronger motivator? Etc, etc.

For me, 'rewards' I have used in the past for various reasons include:

Buy some quality ingredients and cook some delicious food for myself (and/or my family). I like healthy food and a few hours in the kitchen is my idea of a good time

Meet up with a friend to play badminton, tennis or golf. or go for a hike together. It's a workout, but fun!

Swim in the sea.

 

Taco truck sounds good. And bourbon.

 

I think it should be something simple and rewarding - ordering a pizza from your favorite spot or buying yourself a really good beer. It doesn't need to be healthy and it shouldn't fall into this constant self-improvement mindset. It just needs to feel good for you. You don't do this every day - you are doing this because it's a special occasion and you have given yourself the permission to enjoy it.
Cooking a meal for your family or playing sports with your friend are good on their own, but in my opinion they are not really rewards. These are everyday pleasures that you should find time for anyway.