With my indoor quail, they seem to have the most fun with forage-related activities. Sometimes we get dubia roaches from the pet store as a special treat for them. The roaches are more mobile than mealworms and your quail will have to run to catch them which they really seem to enjoy as it probably satisfies a pretty basic instinct for them.
Another thing they seem to like is foraging for seeds, if you have some deeper litter like wood shavings or hay, if you mix a small pinch of bird seeds into it, some of your quail will probably spend quite some time looking for them. As long as you don't exceed the 10% treats rule of course.
That's all I've tried for now but if your quail will eat veggies like cucumber, I bet having a hanging feeder that they have to jump up and peck at would work too, I want to try it myself but my current quail don't like green stuff much yet.
and +1 to all the suggestions of having some branches/hides/cover. Ours love to hang out under the fir branches in the pen. Anytime you can add a different type of natural material like some flat stones, wood, leaves, grass, etc, they seem to like that, its more enrichment for them.