Park bench: an excellent opportunity for some alternating leg step ups. Try some press-ups (on the seat and/or on the back...it will work different sections of your pecs). Do tricep dips using the seat. Standing sideways, do a step up and then kick your outer leg for hip abduction.
Hill: It's a killer but try a hill sprint. Make it your goal to get better at hill sprints. It will improve your anaerobic capacity like nothing else! Also try walking backwards up a hill. There is less impact on the knees and it challenges the brain in the most delightful way.
Tree trunks: make for excellent wall sits. Maintain a 90 degree angle with your legs and "sit" for 60 seconds. Make friends with several trees in your park where you can practice this.
Low-hanging branch: If you can find a branch at about head height grab hold and hang, then pull your knees up to your chest 10 times.
On the ground: If you don't mind getting your hands dirty you could try crab crawling or bear crawling across the grass. Or if you are really feeling on top of your game you could try 10 burpees!
Wishing you a wonderful walk! Let me know how you get on.