Some items and thoughts that should go into every musician's practice room.
A great way to get visual feedback on technique, posture, and anything else. At first it's a little awkward to be looking at yourself in a mirror all the time, but that goes away fast.
Basically THE best practice tool out there. Practice EVERYTHING with it (scales, vibrato, whatever, you-name-it). It always keeps perfect time. Some people think they speed up and slow down, but it's not true--except for the online JamPlay one, it... struggles... Also contrary to popular belief, metronomes DO facilitate "grooving" in general; having a machine keep time does not erase your "groovability", it IMPROVES it by forcing you to mentally categorize notes and note patterns into an accurate rhythmic structure. Then your time will be solid even when playing by yourself.
Recording Device of Some Kind
I use my phone. There are dedicated recorders out there. Whatever will get a you a decent representation of your sound should work fine. It's a similar concept as the mirror, something to get outside of yourself and listen back to and judge as an unbiased third-party. Sometimes I record myself doing vibrato on scales just to make sure my width, frequency, and control are on point. And it works for anything else too, of course.
Pencils, and Maybe Paper Too
Pencils are very important. I like colored ones because there's nothing like using separate colors for each different type of pencil mark (slurs, notes, articulations) and then having your sheet music look like a rainbow exploded on it. The only downside is they're not erasable, so I would actually recommend normal pencils. Write everything down. Stuff that you've accomplished during your practice session will be less likely to be lost if you have a solid way of making sure it stays remembered.
My ideal size for a practice room is 50x25... I like to walk around while I'm practicing. The movement gives my brain extra information to process; it helps prepare for having to focus with distractions--plus it's relaxing to me. However, almost no one has a room that size, but it's still a good idea to give yourself enough space for adequate movement.
Places for Your Equipment
Have spots for your instrument, and any other equipment (metronome, recorder, pencils, etc...) so that they are at the ready.
Reading music off a table is okay, and I used to do it, but after switching to a music stand, an ongoing neck problem cleared itself up because I wasn't hunched over while trying to read my music. It's better for overall playing posture to be able to read music in a comfortable standing position.
Turn your phone off, and anything else that poses a potential distraction.
Try to find an area with zero people-flow as well; practicing by yourself is a less judgmental atmosphere (no one to hear you mess up). Although it might seem like it would be good to have accountability for practicing, it's not quite like that; I've practiced really really well when someone else was listening because I was trying to do a good job at practicing, HOWEVER, I didn't woodshed the parts that I was the worst at, because I was embarrassed for anyone else to hear me work through those... So it's better to have as much privacy as possible.
Have equipment for the places mentioned above. Rosin, string cleaner, bow, pick, pizza, extra strings, sheet music, etc... Have whatever you need right there that way you don't need to interrupt your focus to find it.
Temperature Control Over Your Space
If your room is too cold, it makes it difficult to practice effectively for any length of time. It it's too warm, you have the same problem. If there is any way possible to control the temperature of your room, take advantage of it.
Device with a Biggish Screen
iPad, tablet, laptop, large phone, any of these will work. If it's necessary that you reference a video or recording during your practice, it's good to have one of these at hand. Also, it's helpful to have tutorials right at your fingertips.
There is one item that many might put on this list, but, I'm going to put it in a special category and explain why it most certainly should NOT be an integral part of your practice habits.
I've heard MANY recommendations for practicing with a tuner. I've tried it multiple times thinking it would improve my intonation. a) it didn't improve my intonation, and b) it drove me crazy. So, it didn't improve my intonation, and here's why. Intonation is in the ear. A tuner cannot teach you to hear whether you're in tune or not. To play in tune, you have to learn how to hear when something is in tune or not, AND also be able to adjust on the spot if necessary, neither of which a tuner can teach you to do. So you become reliant on the tuner instead of developing intonation from within yourself, which, if you do that, you'll be able to hear when you're in tune during practice, and a tuner won't be necessary. The adjusting to make the "in-tune line" be in the right place takes your focus off of YOU listening to YOUR OWN intonation, and that's no good in any situation. Tuners are also just enough to drive anyone crazy. Constantly trying to adjust to perfection and then waiting for that device to get a "reading", and then having to adjust again because you were a millisomethingth out of tune and then having to wait for another "reading", and then now you're out of tune the other way so you have to... and so on. So my advice is to learn to hear what "in tune" is, and then hone those skills during your practice, not your tuner-watching skills...
I hope some of these help!