Are there alternate methods to activate Steam CD Keys?

I just looked it up. I ended up on this article.

Important: Ensure that you are activating your game through the Steam application. The Steam website cannot be used to activate a game. If you have not yet done so, you can download and install the Steam application by clicking on the green "Install Steam Now" button here: http://store.steampowered.com/about/

So no, it's not possible to activate games without using the client.

Aside from the client, you can also activate Steam keys through the Steam store website.

https://store.steampowered.com/account/registerkey

At the time of writing, there's no way to get to this page without the direct link, so I suggest bookmarking it.

You can also pre-fill the key like this:

https://store.steampowered.com/account/registerkey?key=AAAAA-BBBBB-CCCCC