Some of the places I've seen folks find success at sometimes...
Their local church gym
Handball / racquetball courts
Local community center gyms
YMCA / YWCA gyms
Indoor soccer fields
A lot of times the tricks just seems to be finding a time slot where any of those aren't being used, and approaching the management correctly with good videos and description so they know it's a real, controlled and safe activity for their space.
Otherwise you can also go urban as suggested above, which could include courtyards and walkways between large buildings that obstruct the wind effectively - of course, onsite security may chase you off (insurance liability mostly) but if you can demonstrate skill sometimes they'll look the other way - if not, be graceful and slide right on out of there.
I kept putting off this reply until "I had more time" but it got away from me...
For quads, I generally rotate through these moves...
LOUD dive stops
Spins near the ground
Walking the kite
- balance on flagpoles, etc
- move objects (flip flops, beach balls, etc)
- landing on things like tents, etc
- landing in someone's hand (with consent)
Catch and throw (on short lines)
On dual line, I'm usually pounding these...
Hard snap landings
LOUD spins near the ground and back up to top of window
Fast combinations (squares, circles, landings)
Making it look easy. Amidst the more complicated and active movements, sometimes flying the kite in such a way that I can relax, look around, maybe sit down, anything to make it look easier and more accessible in-between the intensive stuff.
There's more but that's what comes to mind off hand... My idea is thinking about and developing the things that are most likely to catch (and hold) a spectator's interest, basically if there is a kiteflier inside them, the things that will most likely engage them.
What I've seen in my experience, other than demonstrated skill, the main things that seem to capture folks' attention include...
Tails (one or more)
Stacks / trains
"Defying physics" (like the dive stop)
Incomplete thoughts above, but a start for more discussion.