Friends, I crashed my MTB this week and broke two bones in my left ankle. Luckily I will not need surgery, but I am probably looking at 2-3 months off the bike. I’m on crutches now with a boot, and can hopefully lose the crutches in 6-8 weeks. My main priority is to just recover well, eat healthy, etc., and I understand there will be a significant loss in fitness. But what would you recommend I do over the next 2-3 months to build/maintain as much fitness as possible? I will mostly be stuck at home (where I have some workout bands, but no weights), but can probably swing a gym trip twice a week. My initial thoughts were to work on core strength at home + some machine work when I can get to the gym. Maybe some flexibility work also. I wish I could stress the endurance system a little, but I probably will not have access to a hand cycle or some other upper-body machine. Any other thoughts? Also, how much fitness should I expect to lose? I literally just reached (almost) 4 w/kg for the first time the day of my crash (314 FTP @ 78.9 kg).
HMB supplementation maybe, it’s safe and cheap and might preserve some muscle
I’d focus on recovery, you will be surprised how many calories your body will use recovering from broken bones. Trying to mitigate your fitness loss will only elongate your recovery.
Once recovered you’ll regain your fitness quite quickly 
Theoretically sound advice but in practice I felt awful doing nothing while was in same situation over summer (just 3 weeks off). Even very low intensity exercise would have been better. And I mean physically bad, not just stress. Possibly some kind of dopamine withdrawal or something like that 
Anyway, did not thought about it at that time but now that I have access to SkiErg, I think it is quite easy to imitate using one if you have no access: just use water bottles (a la 1kg) and pretend double poling in sitting position, raising hands instead of pulling with stroke frequency 30-40/min, or whatever feels natural to you. This way you’ll get decent core/shoulder/lats workout and retain some metabolic fitness. But yeah, do not start before inflammation has gone down and as concession to recovery even later, keep HR in Z1 or low Z2. Start maybe with 2-3x 10min intervals and increase it slowly over time depending how your aerobic decoupling improves.
Stationary hand cycle, put your boot on a towel and let it slide with a rower.
Don’t do anything too hard and crazy, just spend some time with your HR up over 100…
Aaaaah I’m so sorry to hear about the crash and resulting injury 
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My main priority is to just recover well, eat healthy, etc.
I think your priorities are straight. Nutrition can play a huge role in your recovery from injury, so make sure to fuel yourself well!
I realise you mentioned that your ability to leave the house will be limited. However, if you can, you might consider swimming. If you can’t kick, you could use a pool buoy to limit the motion of your legs.
You could schedule a triathlon Training Plan in Plan Builder so that there is structure to your swim workouts. The Triathlon Plans schedule between 2-4 swim workouts per week depending on the Plan Volume. You might be surprised at how effective swimming is for cross-training! 
If you have any questions about the Triathlon Plans, please just send them my way!
Sending you healing vibes! All the best with the recovery.
You might want to consult with a physical therapist. You’re going to have changing needs, and abilities, as you move from the initial injury to basic fitness while in the boot to rehabbing your ankle once you can move it. None of those things are all that complicated but pro’s a help with an overall program will get you back as quickly and efficiently as possible.
Buy a rowing ergo!
I would get one, but my wife won’t let me put it in the living room…