Your symptom of morning stiffness may be coincidental to beginning Letrozole as it is not known to cause these problems.  Without more information, such as your age, other medications, including over-the-counter and alternative medications, current status of the cancer, your nutritional status, smoking history, previous or on-going medical problems, I cannot give you a specific answer.  A discussion with first your oncologist, then possibly a rheumatologist or pain medicine consultation may be helpful.

That being said, it is recommended that you be as active as possible.  When the weather is inclement, you could consider walking in a mall or a on a treadmill.  Also, water aerobics is an excellent non-impact exercise that is helpful to many with similar difficulties.  The social aspect can be uplifting as well, especially if you go with a friend.

As far as other treatments you mentioned, I would not do anything without a recommendation from a physician.  It is possible to make pain worse by not treating it appropriately.  There are risks with any treatment we may use.

I hope this is of some help to you.