As the others have indicated, the best return on exercise involves concerted effort, pushing yourself to the limit. That said, casual exercise can also be worthwhile, at least for the purpose of staying relatively healthy. It's not "lying in bed and doing nothing", but it's also not "8x5 strong-lifts of maximum weight" or even "10 push-ups and 10 pull-ups every morning".
First of all, for general exercise, walking is not a bad start. It generally doesn't require much exertion and with the addition of a book, a smart phone game, or a podcast, it can be entertaining at the same time (obligatory note, avoid distractions that prevent you from noticing your environment). If you're willing to set aside the book, you can add a bit more to your workout by making up little games to vary your routine, such as ascending the staircase into the park along the outside instead, traversing the playground equipment however it makes sense at the moment, or even just walking along a curb, trying to balance yourself. It's not intense work, but you get some of the benefits as with the horse-riding of your body having to work to maintain your equilibrium.
Another option is to incorporate some sort of exercise in your times when you're stationary. A stress ball squeezed while reading forum posts online is still exercise (and good for stress relief as you imagine squeezing the necks of commenters). With a small dumb-bell, or even a bag with some books in it, you can do curls with a free hand. Even keeping yourself upright at the edge of the seat instead of leaning back in your chair works core muscles in a fairly passive manner. If you're willing to make an investment into more equipment, there are people who work on a treadmill or exercise bike (or have something under their desk to simulate the same).
Lastly, there's a technique called "greasing the groove" typically used for pushing one's limits, but also usable casually, where you don't bother with a regular workout, but rather just do a few reps whenever the idea occurs to you. You can do a few pull-ups everytime you pass the door frame, do 5 push-ups every time you get up for coffee (pushing up against a wall or your desk if you want to make it easier), or doing a few lunges while your code is compiling. It's low effort, and won't build up muscle, but it's generally painless and it does reinforce the idea to your body that exercise doesn't have to hurt.