I think you came here to see if you need a reality check, and you do. Your toddler isn't "well behaved" at all - because he is NEVER opposed apart from when danger is concerned. But he's refusing to do basic things like get in the buggy, and there are other things he refuses to do, like sitting still for five or ten minutes (ie in a restaurant - I assume you've at least attempted McDonald's or a cafe, explaining that we sit in our chair to eat our lunch, we use our indoor voices, we don't walk around while eating, and stay put until at least had a few bites of the food etc). How do you think he'll go at nursery when nobody has ever insisted that he sit quietly and can have his run-around-play-time later? He'll either be heartbroken at getting into trouble, or defiant and cause problems disrupting the group.
You also can't claim he's well behaved when you have NEVER said no to him on the random toy he picked up in the shop! Try saying your gentle "no" and see if you get resistance. I bet you get a whole load of it. This isn't abnormal in a small kid of course, and he won't fully understand why it isn't his, but it's time he learned that he can't have everything he wants. Do you, the mum, just help yourself to the Mercedes you like in the car park? No, you don't. Because we can't have everything we want. But you, right now, are letting him decide what he wants, without regard to you or to anyone else at all.
There is plenty of space in child-rearing to allow a kid to express themselves and find their way in the world, without having them rule you. I would encourage you that when you go to the park, be loud and boisterous with him, shouting about running about free, jump onto that, give that a try, can he climb this on his own, etc. Give him the space to "be wild" and help him understand there are appropriate places to be noisy and there are appropriate places to settle down. And start with the "we're inside now so this is a place to calm down and keep our shoes off the sofa" type chat.