instead of fitting it out for play, I'd be inclined to make it storage for play equipment. You'd be amazed if you build your toys for purpose and more than one use just how much more you can fit in. Folding equipment and stuff that packs away is your friend when you have kids! If you fit the space out you'll find you tire of it easily as you'll end up limited on positions and ways to use the space as its set up
In stead of another bed as suggested above, try a folding massage table (you can find these secondhand too) they are an easy kit out, all you need is a drill and eye bold attachments. Just put the eye bolts at different points along the framing so it can used in many different ways and positions. Don't forget further down the legs so it can used at ground level too, a set of leather ankle and wrist cuffs and a few Carabiners (buy good quality at sporting shops and they last for decades) and your good to go! if rope you thing it also works for attachment points.
For the bed: buy or make a under mattress attachment set up this one springs to mind if you're a DIY'er
https://www.youtube.com/watch?v=JjMyUWBNOoo&feature=youtu.be
a sport sheet is also another good investment. ExtremeRestrants sells a good one. If not you could make your own with rubber backed curtaining material.....No more lube and fluid stains on your good sheets LOL and your welcome!
A sturdy wooden chair is also VERY easy to modify in the same manner as the massage table above. A slatted back will also give you more use and positions. Some chairs if structurally stable enough, you can cut a "V" into the seat for clit or CBT play. Paint in what color rocks your world.
Jump on line and take look around for a door mounted chin up bar (some are removable and the weight keeps them in place) They are perfect for over head attachment. I love using mine with a spreader bar (easy to make too from broom handle and eyelets) I've fitted the other side of the door frame with eyelets (inside the walk in) so the spreader can also be attached to the door frame for limiting movement and perfect thigh gap ! also works to use a flogger/whip. If you also add them to the inside at the top you can use the points for things like a rope web on the door opening, like the below....or let your imagination run wild.
https://collarncuffs.com/resources/doku.php?id=rope_web
You might/will also need a storage box for tack. I used for years a wooden toybox with a padded top., painted up to match my chair. The inside was fitted out to hold(and hide) all my toys and tools. The box could then be pulled out and the top used as presentation platform (most are good height for being taken or giving head on), a place for the Dom/me to sit, a OTK spanking bench. You can also fit out the base with attachment points for more variations on positions. If you're worried about weight and moving it all the time. A decent set of locking wheels help that. Fill it with things like a Shrews or stocks
https://collarncuffs.com/resources/doku.php?id=shrews_fiddle
If you own the house. Add some decent attachment points to your ceiling (corners work best and save on room) When not in use just hang a hanging plant on them.
Make sure you fit them right
https://collarncuffs.com/resources/doku.php?id=suspension_rigging
Floors are also great, just throw a mat over them. If you put them above and below eachother, a chain between can have many uses.
a stripper pole is another good one!
a simple saw horse is another one, that's easy to fit out or build yourself. Painted up they look good too!
https://collarncuffs.com/resources/doku.php?id=saw_horse
for decades, I had a full sized, free standing StAndrews in my bedroom (7 foot tall) . I had a bottom rail that was fully removable with simple dowel pegs. I then had made an artist canvas that sat on the bottom rail (easy enough to do yourself, can even get the kids to paint it with hand and feet prints) . The Cross was fully covered and actually looked pretty good! But more to the point no one ever guessed it was actually a StA, most assumed it was just an art Easel.
Also if you take a look online or ask around you'll find loads of building plans to BDSM that folds up (for transportation and storage) I do believe there is also a group on Fetlife that shares building plans. Sartans BDSM workshop used to have some great plans too that should still be around.
*edited clean up a link and several typos (coffee needed STAT)