Technically, wherever, except on soft sand. It'll be heavier on the grass, it'll be trickier to go over the curbs. However, you won't be afraid of them in 2 weeks, after mastering the techniques.
COGY is OK to get wet.
Feel free to do water fight. A weather proof seat cover will further reinforce your adventure sprit.
Uneven surface
There is a tip when you go over curb etc. Do not go into it with angle. Go straight. Do not hesitate. Up to 5cm curbs are no problem. In the below photos, left is a bad example, right is good.
Above photos are examples of NO-problem curbs. The man in the red T-shirt had zero hesitation to pedal over these area. He used to get stuck at these type of surface. However, he got better and better every time. At the end, you will find yourself excited to see rough surface. It must be a good training for your brain, too.
The good point of COGY is, you don't have to have a van to take COGY with you (how to load a COGY onto a hatchback video). You don't have to make a precise plan. Just follow your nose, then you'll have the best day ever you can.
walkway / market / shopping mall / museum / library / festival /fun-walk events
You don't have fear anymore, to be tired and get stuck.
Caution!!
Do not pedal down the slope if you cannot pedal up the same slope.
Before you go onto slope, practice pedalling backwards. This is how we slow down on decline slopes. Try not to rely on the brake lever. Use your legs/feet. It is safer that way.
What happens on soft sand? (First of all, please don't even try..)
It'll be extremely heavy to pedal, or impossible. The pedalling will slip. This is actually a very bad sign. The differential gear is receiving huge damages and it may soon break.