As a little critter growing up, I'd spend my play time hanging from monkey bars, crawling up the slide and swinging upside down. These times on the jungle gym with my mates left me feeling happier than a fat kid in a donut shop.
If it were a little more socially acceptable for an adult without any children to be found hanging around a playground, I would do it far more often. However, grabbing some mates and hitting the park together for some fun is not as frowned upon. Throwing in a day outdoors into your training and enjoying the challenge of defying gravity with your own body can be fun and serious at the time time.
Sometimes you need less structure and more play, sometimes you need to just kick back and use the body that you have built.
The 21st century fitness world loves throwing the word functional around, the majority of the training world spends their days picking up heavy things, rowing them, pushing them, pulling them and shouting about how functional their training is.
If this is the case, if you are being 'functional' get outdoors and see what you made of. Put your training and your body to the test. Climb the ropes, dip from the parallel bars, work on your pull ups, row from the low lying bars. There are hundreds of combinations to workout a sound program that will probably give you a harder training session then you would have gotten indoors, playing with the TRX and the bosu ball.
Not only are you challenging your body but you're outdoors, hell, it'll make you feel alive and far less tense afterwards. You're in a park, so there's open space, finish your workout with some field sprints, drop in a higher level of conditioning than you would have received in the gym, on the treadmill, staring into the abyss.
You do not need a membership, spandex tights and the latest Reebok trainers to get in a solid workout. All you need is a playground, your bare feet and the drive to challenge your body. You can still walk away with some serious callasus, a sweat soaked t-shirt and pounding muscles.