Music

It looks like you're planning to record in there, if you're also going to be doing any mixing at all... can't recommend enough getting Sonarworks Reference. I know producers with excellent rooms who also use this software, and if your room has a lot of issues it can be a lifesaver.

Really though your absolute first port of call should be getting as much acoustic treatment as you can afford up. Bass traps in the corners, acoustic panels on the walls (if you buy ones made specifically for acoustics and cover enough space [don't forget ceiling] they will tamp down high reflections / that 'spangy' sound when you clap as a test). All your recordings will sound like a fart pushed through an onion without treatment. If it's a rental and you're worried about wrecking the paint sticking things to it, make a 'string' of acoustic panels (duct tape the string to the middle of the back of the panel), then you can put two thumbtacks in either end of a wall, tightly tie the string to each end, now you've got a row of acoustic panels with only a couple of pinpricks in the paint.

You can make a cheap 'isolator' if you want to make a vocal booth-esque thing, by draping a heavy duvet over a clothes horse, then positioning that around whatever instrument you're recording.

Is that a cheeky pair of yammies monitors I see by the door? :)

To make it less small, consider knocking out a wall. The one between the drums and keyboard would really open things up. But for real, I'm not sure there's a great way to do that.

You might want to look into building some sound absorbers to put on the walls. Don't but the acoustic foam stuff, it doesn't do much and is expensive. You can get semirigid fiberglass boards fairly cheaply and wrap them in fabric, then mount them to the walls. There are a bunch of tutorials on YouTube.

Nice studio! Well done on the spacing so far. Things that helped me a lot in a smaller studio was utilizing the walls, and having dedicated spaces for everything. Our walls have hangers for some instruments, gig bags, and low hooks for frequently used cables. We also use an old dresser with drawer dividers to keep gear organized and away from dust. One bookcase for sheet music, set lists, with the top shelf for the metronome, and little bowls for picks. Every year there seems to be more gear and cables, so having a specific home for everything keeps it neat.