My first suggestion would be cycling, but that's mostly because that's what I love.  It's impact free, and you can take it as low key as you want.  The one thing to watch for is your knees.  Make sure your seat is correctly adjusted for height.  To heigh or (more likely) too low will put stress on your knee, which are already getting used to running.  Just go for a gentle ride to keep the legs moving.  As your fitness improves, you can turn it into real cross training.  But to start, just have fun.  And I don't mean to scare you with the knee thing, it's just the one area to be aware of.

Swimming is also good in that it will work different muscles, but is even more stress free than cycling.

What about Yoga?  Frisbee?  Walk in the park?

I think the main criteria should be 1) you enjoy it 2) you enjoy it 3) it's low impact

p.s. HUGE congratulations on quitting smoking and getting out and moving!