How to get a decent workout on a mid sized, flat parking lot?
Consider constructing a parachute similar to those worn by sprinters: https://www.amazon.com/SKLZ-Speed-Chute-Resistance-Acceleration/dp/B002CLK21C

This would generate significant drag at moderate speeds.  Just be sure not to get it tangled up in the rear wheel!  Perhaps install an emergency release to avoid getting tangled yourself.  It's been discussed here: https://www.bikeforums.net/33-road-bike-racing/273066-anyone-ever-use-sprinter-s-parachute-added-resistance.html

Another option is to wear bulky clothes that would act as a parachute/sail.  I often achieve this effect unintentionally with rain gear.  Throw your bike into high gear and grind away!