This is a small tip coming out from my real-world experience. As you know, activities can be browsed either by keyboard (Ctrl-Alt-N and Ctrl-Alt-P), by zooming in and out (using the cashew) or by using the "Activity Bar" applet. And it's the activity bar we'll use, combined with some simple Plasma magic, to make switching easy.

1. Create a new panel and put it where you want
2. Add the activity bar to said panel
3. Resize the panel to your liking so that it's the size of said activity bar
4. Activate auto-hide for the panel

Voila'! You can just hover to where your panel is hidden then select your activity. Here's an example from my EeePC (activity bar on top; click for larger image):
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