This is an amazing resource from the MAGE center at Macalester. Click on each of the sessions at the top to see a list of free curriculum resources on Social Studies themes. Also look for opportunities to do free education forums with this group. Very useful materials!
Labor Day is an excellent time to dig into social studies themes civic ideas, power authority and governance, and production/distribution of goods are all relevant. What will you do?
This site has units and lesson plans on: Lewis and Clark preparing for the trip, politics, mapping, women, animals, language, trade and property, plants. There is a section on exploring and using primary and secondary sources.
I labeled this generically with the NCSS label as webquests will be available in every topic area you could want in Social Studies. It appears this has some useful tools for helping design your own webquests.