I have 4 year old twins and I always try to teach them to slow down and take a look at what is going on around them. Yesterday, we were walking back from the grocery store to the car and they heard a cicada. So I told them we should go on a "hunt" for it. We followed the sound to a small tree and I got them up on my shoulders so they could find it.
They are constantly stopping to look at bugs, birds, clouds, or "fossils" (aka rocks). It takes forever for us to go anywhere, but it's worth it to see them discover the world around them.