Hi, there,
I have the small size of The Stick massager. A PT recommended it when I was having issues with my IT Band. I giggle whenever I take it out of its tube--the tagline underneath its title says, "A toothbrush for muscles!" If you don't overdo it, I think it works well~
Also, regarding your knee getting stiff when you are "just" lying around, I agree with
@sistersinhim. Let your knee be your guide. Unfortunately, MY knee curtailed my activities for quite a while! Sometimes rest IS what it needs. When I was under forced inactivity, I did a lot of chair stretches and muscle relaxation. These helped with the restlessness and also kept my knee from locking up with stiffness.
One stretch I did was to extend my leg and raise it as high as I could (while reclining in my recliner), then loop a towel under my foot and hold as long as it feels good. It seemed to help a little bit with extension, too. Of course, bending and flexing your knee while sitting helps with stiffness, also. Nothing strenuous--with leg elevated a bit off the chair, just bend gently and then straighten.
Regarding relaxing the muscles, there is a technique called Progressive Muscle Relaxation that is recommended on many wellness sites, including the Mayo Clinic and Cleveland Clinic. Hopefully it's okay to post a couple of links. The first is a "how-to" description in PDF from Anxiety BC in Canada, and the second page, on Cleveland Clinic's website, has a link to guided audio instructions partway down the page.
https://www.anxietybc.com/sites/default/files/MuscleRelaxation.pdf
https://my.clevelandclinic.org/health/articles/11887-stress-learning-to-relax
BoneSmart probably also has something on this. I tried the Search function but evidently wasn't putting in the right combination of terms. Personally, I think Search hates me.
(Just kidding!)