As with many other technologies, the design time experience in Windows Workflow Foundation uses many of the same extensibility mechanisms found in other designers (such as the Windows Forms designer). One of those is the hability to provide custom type editors for properties by creating an UITypeEditor-derived class and then associating it with the property via the [Editor] attribute.

Here's a sample UITypeEditor that will present an OpenFileDialog to choose a file name as the property value:

//
// FileSelectorTypeEditor.cs
//
// Author:
//    Tomas Restrepo (tomasr@mvps.org)
//

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.ComponentModel.Design;
using System.Drawing;
using System.Drawing.Design;
using System.IO;
using System.Text;
using System.Windows.Forms;
using System.Windows.Forms.Design;

namespace Winterdom.Design.TypeEditors
{
   /// <summary>
   /// Customer UITypeEditor that pops up a
   /// file selector dialog
   /// </summary>
   public class FileSelectorTypeEditor : UITypeEditor
   {
      public override UITypeEditorEditStyle GetEditStyle(ITypeDescriptorContext context)
      {
         if ( context == null || context.Instance == null )
            return base.GetEditStyle(context);
         return UITypeEditorEditStyle.Modal;
      }

      public override object EditValue(ITypeDescriptorContext context, IServiceProvider provider, object value)
      {
         IWindowsFormsEditorService editorService;

         if ( context == null || context.Instance == null || provider == null )
            return value;

         try
         {
            // get the editor service, just like in windows forms
            editorService = (IWindowsFormsEditorService)
               provider.GetService(typeof(IWindowsFormsEditorService));

            OpenFileDialog dlg = new OpenFileDialog();
            dlg.Filter = "All Files (*.*)|*.*";
            dlg.CheckFileExists = true;

            string filename = (string)value;
            if ( !File.Exists(filename) )
               filename = null;
            dlg.FileName = filename;

            using ( dlg )
            {
               DialogResult res = dlg.ShowDialog();
               if ( res == DialogResult.OK )
               {
                  filename = dlg.FileName;
               }
            }
            return filename;

         } finally
         {
            editorService = null;
         }
      }
   } // class FileSelectorTypeEditor

} // namespace Winterdom.Design.TypeEditors

You can then easily associate it with a property like this:

[Editor(typeof(FileSelectorTypeEditor), typeof(UITypeEditor))]
public string Filename
{
   get { return _filename; }
   set { _filename = value; }
}

One thing to watch out for, though, is that at least with the July CTP of WinFX, WF will completely ignore your custom UITypeEditor if it is defined in the same assembly as the activity using it. Don't know why, but make sure you define it in a separate assembly you reference, and it should work OK.