Yeah, you can pretty much see Bldr Falls from the highway ;)
Your son might have fun flying kits at Chatauqua if it's windy. There are also great hiking trails at Chatauqua- easy/moderate and right up to the base of the Flatirons. It's neat to hike up there and watch the rock climbers do their thing.
If you have bikes, you could bike the Boulder Creek Path- which goes right near downtown.
There are occasionally festivals and such downtown, maybe check the schedule for when you are here? getboulder.com/things_todo/todo_attract.html
That site also talks about NCAR- I grew up right below that building, it is a neat place to visit too- great views and hiking.
There are a couple unique restaurants that I love around town. The Med is a locals favorite. They have a wonderful patio. The West End Tavern is right on Pearl street and they have a fantastic rooftop deck if it's not too hot that day ;) My favorite place is 14th St. bar and grill at 14th/Pearl. Be warned - kind of exotic art on the walls!
Another local institution is Turleys at 28th/Pearl. It's not 'on the mall' but it's the place where you can get your soy latte and tofu scramble type food- you can also get a burger (natural beef, of course!)