I'm going to address your post in two parts. The first part will discuss the CMS options out there. The second part will be a suggestion for you to consider in terms of easy installation and hosting.
Part 1: CMS
I've used a ton of CMS' and designed custom templates for each of them. Here are my top 3:
1) WordPress. What's what? WordPress is a blog, not a CMS? Afraid not, chum - WordPress is so much more than a blog. Although it's not the most powerful content management system out there, it's my favorite for simple sites that you describe. The admin interface is intuitive - perhaps the most intuitive I've ever seen. The documentation is second to none. The user community is strong and PACKED with useful addons. At the moment, despite some serious problems with TinyMCE, WordPress is my favorite development platform. The templating system is one of my favorite because it's so easy to manipulate on a custom level. It's probably not the most elegant, but who cares if it works well? Definitely check this out.
2) Drupal. Like Josh mentions, Drupal is a great piece of software. I spent about 5 straight days developing an entire website on Drupal, and although I was impressed with it, I was also constantly frustrated by minor issues. The biggest problem I had was that I didn't know PHP, which made it pretty difficult to extend / manipulate extensions the way I wanted. Personally, WordPress was more simple in this way, and with a better set of extension resources. I also prefer WordPress' theming system. In my opinion, for simple content management, WordPress is better than Drupal. Josh probably feels differently
3) Plone. Plone is serious stuff. It's written in Python and based on Zope, which means it'll cost you about $4 billion dollars / week to host it, since it's so processor intensive. The purists love it and would scoff at the idea of using anything else. If you're up for a challenge and know Python or are familiar with Zope, go for it. Otherwise, stay away. Stay far, far away.
There are many, many other options. The most important thing for you is to choose one and learn it inside out. That will allow you to train your customers to use it quickly and effectively, and allow for rapid deployment. I've chosen WordPress and I'm sticking with it - find what works for you. See part 2 of my post for a good way to experiment with these.
Part 2: A good way to play with all of these, and to deploy these systems to your clients.
Some hosts out there offer Cpanel and Fantastico. The combination allows you to install all of these applications with the click of a mouse, which I love because I'm lazy and hate terminals. Some hosts offer reselling, which allows you to setup multiple, independent cpanel accounts for all of your clients, on different databases. The only host that I would ever go with that offers both of these is
a small orange. I believe
Dreamhost offers reselling, but i don't think they offer cpanel. I could be wrong about both of those. [PS if you do sign up with asmallorange, I'd love a referral (
desenso)]. What does all of this mean? With a host that has fantastico you can try out 10 CMS' in a day and see which one you like. Then, you can deploy installations to your clients' websites, build them a custom theme, and give them a basic lesson on how to use the software to do the things they want to do. In my opinion, this would be the most professional solution to your problem, and be a great way to expand your skill set.