Get some low profile "night stands." I recommend little foot stools, which will be useful even after they're replaced with "real" furniture. Apple crates are another go-to. So is an upside-down waste basket (wicker or wood). Having nightstands will make the space both feel more intentional AND more functional.

Style your bed. Without a bed frame, try to make sure your sheets, comforter, pillows etc. all look intentional and put-together. Get matching pillow cases, add a throw pillow or two, and place a folded throw blanket across it. Here's additional advice on bed styling.

Get a canopy. Mosquito net canopies are cheap and can be hung temporarily with command hooks, but they'll make your bed space look both "finished" and draw the eye up. You can get a "four poster" canopy or a round one to drape where a headboard would go. If you want a more substantial look, you can spend a little more and get a fabric canopy.

Hang art. If you have a large art piece, or can make a gallery wall, hang art. This will also draw the eye up and make the space feel intentional. Here art some ideas for adding decor above your bed. You can even add a removable decal behind your bed as a faux-headboard.