Fitting movement into your day can be as simple as going for a walk. But if you're looking for a bit more you can supersize your walk by adding strength exercises to it. As long as you have a safe place to walk, you can stop and do pushups on a park bench. If you come across a hill you can lunge up the hill. I personally like a fartlek technique for lunges or any lower-body exercise. You can pick a landmark about 30 feet ahead of you, or however long you think you can go. Then do the exercise for that distance. Squats can be done this way by taking a step between repetitions.

I love doing exercises on benches or any sturdy raised surface. Some of my favorite exercises to incorporate into a walk are pushups, tricep dips, any variation of a plank or step ups. You only need to do one set of 10 reps of an exercise to benefit. The possibilities and combination are endless. Even better is to recruit a buddy to walk with you!
Choose your goal before you start walking. Decide how many exercises and how many you'll do. A good place to start is choosing 4 exercises and do 2 sets of 10 reps. You can sprinkle the exercises throughout your walk or find a safe place to stop and bust out your circuit.
Here is a list of exercises to get you started (do 1-3 sets of 10-15 reps):
pushups
tricep dips
plank (variations: side plank, shoulder touch, sideways walking plank, mountain climbers, reverse plank)
squats (variations: single leg squat, side walking squat)
Lunges (variations: walking lunge, side lunge, curtsy lunge, back leg on bench)
Have a good walk!