Creating variety while using the treadmill, using weights to increase your metabolism

I have been using my treadmill desk workstation for over 3 months now.

I have lost 4 pounds! This is with no change to my diet and no other exercise whatsoever.
I only use it around 2-3 hours a day. Now that the novelty has worn off- I was trying to find ways to
actually speed up my metabolism and add some variety to the experience.

One of the things I started doing was lifting 3 pounds weights while I read things, like the news online. I have these from Amazon and I really like them. The are heavier then you think after you start using them for awhile.

I have done this kind of exercise on and off for years and it always helps my arms firm up rather quickly.
I have the typical issues with my arms that those of us at the tail end of the baby boomers may be finding at this point. I barely squeaked into that group known as the “baby boomers” but according to Wiki I still fit! I wasn’t sure if I was Gen X, but I guess 1964 is the cut off for baby boomers.

Anyway, one day I was driving and my teenage son looked at me and said “Eww Mom, your arms are flabby”, this was a shock, since I am not super out of shape and really did not think my arms were that bad.
Nothing like having a teenager to tell you the cold hard facts!
Lifting the small weights while using the treadmill desk, is easy, fast and defintely breaks up the routine.
I also like to walk backwards for about 5 minutes while talking on the phone or to my kids.
What do you do to break up your routine?
Why is exercise so monotonous anyway? That is why I like the treadmill desk, I get to multi-task while using it.

Let me know what you are doing with your treadmill!