Never use your hands and arms to lower you into, or raise you out of, a sitting position.

Park as far as possible from your destination (taking safety in mind). Two added bonuses is there are usually plenty of spots, and getting out of the lot is easier.

If it is less than a fifteen minute walk to where you are going then walk instead of drive. You will get there faster than driving will get you there too.
Getting a bicycle is the next step. A friend who commutese by bike now started just by using her bike to go shopping for groceries 4 blocks away. (get a bike that isa bit upright to ride with a good basket.

Get in the habit of using a small packsack when you walk.
It is surprising how fast your ability to carry stuff increases and it is good for most of the body, not just the legs.

Take the stairs.

Carry a grocery bag or basket instead of using a cart.

Walk the dog everyday, good for you and the mutt. Of course you can get trapped into building the perfect dog walking EDC and then there's the dog's backpack, his cold weather overcoat and the booties for when people salt their sidewalks like they do their french fries, and the GPS for making maps of where you've been and the cool, glow in the dark leashes...

Can't figure out why my family starts lauhging when I announce that I'm taking the doog for a walk.

Before getting up from a sitting position, make it a habit to raise your feet off the ground one inch and hold them there for ten seconds, increasing the time as your fitness indicates.

I'm told ankle weights turn a walk into the equivalent of a run.
The Sock

Bike commuting rocks! You have to get to work, you need regular exercise, so riding the bicycle is productive multitasking. A key consideration is having facilities at work place to store the bike and clean up, but this can be arranged with a little ingenuity.

I have been a regular bike commuter since 1970, riding from 22 to 10 miles round trip.

I guess it's not really a "habit" in the sense of walking more or taking the stairs... but, for a while now, I have been doing a stretching routine, and 40 each of push-ups and sit-ups every morning.
Really helps me "limber up" and get a good start on the day.