One of the things that many birders do to document their birdwatching is keep a life list. A life list is basically a checklist of species of wild birds that someone has seen.
Jove and I have both started our life lists. For mine, I printed out a checklist from the front of a bird guide of the approximately 800 birds found in Eastern North America. For the birds I see everyday like House Sparrows and American Robins, I just checked the box and for less common birds I am recording the date and location that I saw it. Jove's list is a small notebook we made where he has written down the names of the birds he has both seen and knows how to identify. Jove's booklet is really more of a self-made bird guide than a life list, but I feel like we can wait until he is a little older for him to maintain his own life list. I also keep anecdotal journal entries about birds that we see: their field marks, behavior and the weather conditions and location where we saw them. I am not including birds that I know I have seen in the past, although I am not sure if you are supposed to or not.
Field marks, such as colors, patterns, shape or size of a bird's head, beak, feathers, etc are what birders use to positively identify a bird. You probably already know what a robin looks like, but how do you know what a robin is? The rust colored breast feathers, the white tips on its wings when it flies, its yellow beak, its size, its hopping behavior. It is useful to notice the field marks of common birds to get used to knowing them on uncommon birds. Field guides include color drawings of birds that emphasize field marks that allow you to distinguish between birds that could easily be confused.
Here's my life list as of right now:
Rock Dove (pigeons)
Jove told me we should keep a life list for everything, not just birds.