I have two recommendations for you that will do what you want. The first idea is to use a simplified CMS like http://www.iUpdateIt.com. This will allow you to install a small snippet of code into their site where you want them to be able to update. Then, they'll be able to login and change that space using an easy editing tool that works a little like a word processing program. It's simple to use and allows the client lots of flexibility to add fonted text, images, etc... The other idea is to get them a blog on something like http://www.Blogger.com, and then feed the content into their site using something like http://www.FeedBurner.com or http://www.WidgetBox.com. This would be more if they want to do stuff like articles, but it works well. Both ideas are easy to create and will be a breeze for your clients to use.
As to the Flash elements, neither of what I said really intergrates as a Flash element, but probably could fit into a "space" on a Flash site... I think that you can do what you want for your clients with either of them.