When walking briskly for your cardio workout, you can challenge yourself by stopping every five minutes and using something in your environment for an extra cardio boost. For example, do front or side legs kicks and tap the tree trunk gently with your toes each time you kick out or find a flat stone to do dips or pushups on.
I have been to several walking trails in Maryland that have bars or posts to add a little something to your workout. While sometimes I am not too sure what to do with them (are they for pull-ups? stretching? flexed arm hang?) I appreciate the opportunity to stop and do something different. If you are in a city environment, maybe you can do stretches against a sturdy street sign or ham string stretches against a park bench.