Put window fans upstairs with the fan pointing outside to exhaust hot air and window fans facing in in the evening downstairs to pull cooler air in.
Before AC, most homes in the South had attic exhaust fans. Running those in the evening with the windows open as temps cooled removed the hot air from the home and pulled in cooler air.
There are some window units that are easily picked up. I have central HVAC but my craft room gets much hotter than others so I have a small window unit in there for very hot days.