If you know, or can find, the portions of your floor that can bear the load, you could build another floor a few feet up from yours with floor joists and posts, much like building the floor of a building. Consult a contractor/architect to find out the size and number of support posts/joists needed. Make a floor joist frame to fit the desired area. Attach to posts, include braces for stability. Fill in with more joists as needed. Lay flooring of your choice. Put up a safety rail and or curtain. Build a small staircase/ladder for access. Figure out a way to have electrical power/lighting up there. Maybe, cut all the parts at another location, use bolts for attaching to posts, and screws instead of nails (quieter, less likely to attract landlord than hammering nails/spikes). Oh yeah, be sure to have a fireman's pole, one, it'll be fun to slide down on, two, your significant other can dance with it.