I agree with Internet Human's approach, I think physical baffling is the way to go. I have found success in the past by actually using the floor of my previous house. I needed to record footsteps (and a fight) from above, so I set up 4 mics in the basement facing upwards about a foot below the ceiling, and then I had fun running around upstairs, slamming my body down, kicking furniture, etc.
If you can access a house (particularly a wooden one) with such a basement, try recording a few impulse responses through the floor. What this will get you is a feeling of resonance and size that smaller baffling can't really duplicate. A large, wide floor, especially in an older house with less rigid supports, can act like a "diaphragm" of sorts and resonate low/low-mid frequencies quite well. It might take a little location scouting, but I bet the results would be worth it.
You can also try "worldizing" (re-recording through a speaker in a real acoustic space) your sounds in various interesting acoustic spaces. I'd go for a stereo recording at a minimum, but multichannel would be even better. A few that come to mind:
- put the mics in a kitchen cabinet and the monitor on the counter above (and vice versa)
- put the monitor in your closet and the mics outside (and v/v)
- put the mics underneath your bed and the monitor on top (and v/v)
If you have Altiverb or access to it, the "Next Door" set of impulse responses is quite reliable for these kinds of sounds. You can always blend back in the original sounds in the mix for clarity.
Best of luck!