Walking is fine, I walk nearly everyday, but it does nothing for flexibility or balance.
I do Tai Chi. it is easy to do and we hold on to the back of a chair if we find balancing on one foot difficult.It is very good for flexibility and balance. Lack of flexibility, to look over your shoulder, or twist your neck, can be a reason for losing your driving licence.
I also do pilates, but that is quite strenuous, although some places have pilates for older people or sitting pilates for those whose capacity for activity is severely compromised.