Need some ideas on decorating a small flat metal trellis with lights,

Ooh, an empty frame! Its obvious function is as a garden trellis. This would be a terrific backdrop for a fairy garden. If you could wrap the whole trellis in LED tiny lights, you could add seasonal decorations. The trellis would look good in a rectangular planter, with real or artificial plants filling the planter. A planter would hide the wires to plug in twinkle lights, and you could choose several succulents. Or try a "scrap garden", made of white or sweet potato vines, carrot or radish tops, bean sprouts, pineapple tops or avocado seeds. And don't forget, you can paint the trellis any color or metal you want. Best wishes!

try solar Christmas lights. they would look great and you would be able to put them anywhere without worrying about a plug

I use solar lights on mine

Fairy string lights would work great on something like this.