The MSDN Library folks have posted a series of "Technology Samples" for Windows Workflow Foundation, geared around Visual Studio 2008/.NET Framework 3.5. I have not looked at all of the examples, but if you are using Visual Studio 2005/.NET Framework 3.0, then the majority of them will work with those versions as well.
The one of particular interest is the Designer Hosting Sample, which shows you how to build a UI to allow users to customize/modify workflow's using "only" the shapes that you gave them and with constraints/rules that you can setup to restrict how activities/shapes can connect.
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With each example, you download the project (self-extracting executable) and you will find the Visual Studio solution and limited documentation that you can use to learn the concept. My suggestion is that you download the Workflow SDK as a backup for the documentation.
Here's the list of the samples (taken from the following MSDN page: Click here to access):
Activities Samples
Contains samples that demonstrate how to use the activities that are provided with Windows Workflow Foundation.
Contains samples that demonstrate how to bind activity dependency properties to workflow properties.
Contains samples that illustrate how to create and use different types of workflow's in Windows Workflow Foundation.
Contains samples that show the different activities that are available for object communication between workflow's and other objects.
Contains samples that demonstrate how to create custom activities.
Contains samples that show how to host the Windows Workflow Foundation designer.
Contains samples that demonstrate the ability to dynamically update a running workflow.
Contains samples that demonstrate activities that are associated with hosting the Windows Workflow Foundation runtime engine.
Contains samples that illustrate how to compile a workflow code file at run time.
Contains samples that demonstrate how to work with workflow's that are defined as XAML (Extensible Application Markup Language).
Contains samples that demonstrate the various details that are involved when you create and run workflow's that use roles.
Contains samples that illustrate how to work with rules and use policies in workflow's.
Contains samples that illustrate the different ways to use tracking and create custom tracking solutions.
Contains samples that show the different ways to use transactions in a workflow.
~ Robert Shelton