Here is a video of fish in the fish ladder:
We love taking visitors here and have gone many times. It lets folks get up close and personal with all sorts of salmon. We took our little guy who was only months old, and we were able to get him close enough to the window to see the fish going by. He was following them, so even at that age it was interesting.
You learn about the fish counting (it is basically a person, who sits in a dark room, counting fish all day until someone comes and relieves them). On occasion sea lions make their way up the river and in to the fish ladder (this has been a big problem, and has stirred a lot of controversy when it comes to removing the animals to protect the fish). Kids also get to learn about the life cycles of the fish and what they go through to spawn. There is a neat museum upstairs that talks about Lewis and Clark's expedition and how the Northwest came to be and there is a small exhibit about dams and how power is made with them. There are also locks here, so there is an explanation on how they work as well. Pretty cool!
You have to go through security to get onto the dam (it isn't that big a deal though...you are asked some questions, they check your trunk and underneath the car, and send you on your way).
If you are in Portland or traveling up HWY 84 in the Gorge, this is a really good stop (especially if you need to get out and stretch your legs). It is best to go when the salmon are running too (slightly less to see when they aren't). While you are here, you can also check out the fish hatchery. That is good fun also!