What to fill in a large, 6x8, 3 ft deep,in-ground, cement lined pond?

Gravel and then about a foot of top soil to be able to plant grass, flower bed or garden.

I was going to suggest the same thing...put in a garden. But I would drill several holes through the bottom of the concrete so that it won't hold water. I see you live in Salt Lake...what about making a defined rock garden with a few Yucca plants and a couple of nice resin pots that you fill with seasonal flowers?

Trampoline - Lots of drainage blocks etc. plus soil for a Bog Garden. Or Add steps down for a sunken seating area, but remember to form drainage. OR could use it for building a Greenhouse on top with extra growing room or as a Bigger Summerhouse with Basement area for storage etc.