I would say start doing what you used to love doing, whether it be walking the dogs in the afternoons or hiking a favourite short route along a creek in a forest or going to a beach to walk along the shore. I find being in nature made me feel more 'normal' again.
I did go to the local pool 3 times a week for a easy swim while I was going through chemo, this was extremely invigorating and made me feel in controll. I avoided the weeks my immunity was lowest and also avoided getting my head under the water. I could'nt swim during rads, but was back at it after treatment ended. When I got stronger I enrolled in PT for a couple of months and this too made me feel like I was getting back on track. I since stopped PT because i just can't afford it but have enrolled in the online bone clinic exercised classes (Onero) which I do 3 times a week to help build stronger bones (due to osteoperosis). This is a two year program and as you progress they send you the new videos over a period of 2 years to help you gradually build up your strenght. All in the comfort of your own home.
I guess everyone will be different but do it gradually and start with what you loved doing before. Maybe even look into doing something you never tried before, like join a dragon boat team or a gym. Congratulations on finishing active treatment and all the best for the future x