I don't think there is a "low effort, low impact, easily reverseable" way. I have done research but have not tried these. Browse the audiophile "home theater" boards for more education.

This is what I would investigate.

1. Double-up the sheetrock with spongy "green glue" in between. There is a cheap version of "quietrock" for sale at some home improvement stores. However, "plain-old-sheetrock-doubled-up-with-spongy-glue-in-between" would outperform a single sheet of quietrock.

2. Install a double door meaning within the single-width frame there are 2 doors you must go through. The first opens out into the hallway. The second opens in into the room. The deeper door frame covers the additional sheetrock. Use solid core doors. You see this a lot in doctor offices to block the outside from hearing the conversation inside the room.

3. Install heavy curtain rods the entire length of each wall and hang soundproofing curtains over the walls.

4. Use a heavy/thick area rug with anti-vibration mat(s) as a second layer of carpet underlayment in addition to your typical anti-slip carpet underlayment. Double-up on carpet.

5. I know you don't want to hang stuff on walls but google "picture rails" and install a picture rail on all of the walls. Instead of knocking holes in the wall (or gluing), hang acoustic tiles off of the picture rail. You may find picture rails to be so cool you install them in your entire house. :-)

6. Investigate the insulation around your electrical and HVAC. Sometimes home builders put outlet boxes in adjoining rooms directly across from each other meaning there is no "wall" between the 2 rooms where the outlet boxes are. It's like a big hole in your otherwise effective sound proofing.

(P.S. I really like the soundproof divider idea above. You can probably find used ones at a used office furnature store.)