I'm a big fan of shrimp. Which oddly, you should buy frozen - they will be fresher.

Defrost.

Toss in a vat of boiling water with spices, a lemon etc. With shell on, cook 4 minutes. Will be perfect.

Or toss in a pan with butter and/or oil. Cook until pink and the texture has changed- about the same time period.

They are easy enough to break into small portions, so you can try different approaches without spending a fortune.

For fish? I've had good luck just baking it.