Yes, but it involves a few more steps depending on the design of your Salesforce org. 

If you have an Activity roll-up summary field on Leads, and you've created a formula field "Days Since Created Date" you could accomplish this objective by setting up your Workflow Rule just like this:

      1. Create an Email Template to support this Email Alert (write down the name of this template so that you can find easily when you set up the Alert.
      2. Click on New, then select Leads" for your Workflow Rule
      3. Name your Workflow Rule Lead "7 Day Alert - No Activities"
      4. Add the following two filters:
        1. Total Number of Completed Tasks is GREATER THAN "1"
        2.  Days Since Created Date Equals "7"
      5. Choose Option # 2 on your Evaluation Criteria i.e. Created, or Edited ....
      6. Select "Email Alert" for your Workflow Action
      7. Name your Workflow Action "Lead Owner Alert" (this is important because this Alert is reusable by other Workflow Rules, Naming it this way makes easy to find when creating other rules. )
      8. Select the name of the email template that you created for this Email Alert
      9. Select "Owner ID" on the Recipient Type
      10. Go to "Available Recipients" then highlight  "Owner ID" Click on "Add" to set the owner to the the "Selected Recipients" list
      11. Activate your Workflow Rule
      12. Create A Custom List View with the same two filters used in this Workflow Rule

You can automatically product Activity roll-up summary values on any object with one of our other Apps: Activity Counter.