In the top left of the Activity window is a search field for finding an Activity by name. Select an Activity from the list, then click the Unlock Widgets button in the far right of the window, and you can modify the Activity just as you did the default desktop.
You can also click on the wrench icon in the bottom right of an existing Activity listing to change its name or icon. By contrast, click the square in the top right to disable and remove an Activity.
To add an Activity, click the Create Activity Button on the far right of the window. From there, you can clone the current Activity, use a template, or create a completely fresh Activity. The templates sub-menu includes a link from which you can download new templates, although it is marred by a lack of previews and inadequate descriptions -- to say nothing of a lack of selection.
Some of these templates give you the option of starting applications when you switch to them. For instance, the Photo Activity template offers to start the digiKam photo manager.
(Incidentally, two of the templates, Search and Launch and Newspaper Layout, may be familiar to you if you ever tried Plasma Netbook, KDE's short-lived offering for netbooks. Since KDE separates the interface from the other functions of the desktop, developing a shell for individual hardware and settings is a relatively easy bit of coding.)
When you have created the new Activity, customize it as you please. Just remember to click Lock Widgets when you finish, or else each icon and widget will trail a small icon sidebar that will only serve as a minor annoyance.
Once all your Activities are set up, you have several ways to switch Activities. Besides selecting Activities from the desktop tool kit, you can select Activities from any desktop's context menu, or press Metakey+Q or Alt+D followed by Alt+A to open the Activity window and select with the mouse.
To switch to the next Activity, press Metakey+Tab, and to switch to the previous Activity, Metakey + Shift + Tab. You might also get into the habit of adding the Activities or Activity Bar widget to each Activity.
However, one of the most convenient ways to switch Activities is to select Desktop Toolkit -> Desktop Settings -> Mouse Actions, and assign a combination of buttons for switching Activities. Personally, I find reassigning the scroll wheel from switching Virtual Desktops to switching Activities most convenient, but obviously whether you agree with me will depend on whether you make much use of Virtual Desktops.
A few sessions with KDE Activities soon convinces many people of their possibilities. Yet, as the relative lack of templates suggests, the interest in Activities by KDE users seems disproportionately small compared to their usefulness.
Why? Some users may simply not want to take the time to set up Activities to their liking. Especially if they rarely multi-task, a plain traditional desktop may be all they need or want. Some may not care to re-orient their thinking to task-based organization. Still others, familiar with recent GNOME releases, may be confused about what KDE's Activities are about.
All these factors may contribute to the obscurity of Activities. However, the largest reason seems to be that KDE has simply failed to market the concept well. If you dig, you can find some useful explanations of Activities -- notably, Chani Armitage's blog -- but no easily accessible documentation.
Nor is the situation helped by the confusion between Virtual Desktops and Activities in the interface, or the three different methods that have been used to display Activities. Even templates add to the confusion, having been previously known as views and containments. While the concept has remained the same throughout the KDE 4 series, the presentation has been so inconsistent that I suspect that many potential users decided the whole topic wasn't worth exploration.
To say the least, this situation is unfortunate. In Activities, KDE offers an elegant extension of the basic concept of the desktop. If you've shied away from exploring Activities, you may be surprised at how simple to use and efficient they actually are.