Depends how you are feeling and your willpower.

1. Easiest thing you can do is meditate. Sit up straight, but relaxed. Set a timer so you don’t have to think about when it’s time to stop. Focus your attention only on your breath. Inevitably, your mind will wander, that’s okay, just forgive yourself and return attention back to your breath. When you focus on your breath, don’t focus on controlling your breathing, just observe it passively.
2. Next easiest, look into Wim Hof breathing. This will be a much faster breathing exercise and will give you a lot of energy, but maybe it’s a little difficult to follow compared to simple meditation and breathing.
3. Take a cold shower or go for a swim. It’s hard to find the motivation to do this, but after a cold shower, you feel amazing. It’s like you just did a restart on your physiology.
4. Exercise. Get your heart rate up. Just a simple jog or brisk walk is good enough.

Also, don’t spend too much time lying in bed, especially if that’s where you sleep. Find a comfortable chair or couch instead.