You may want to consider getting married in a Quaker Meeting House. You do not have to be Quaker to get married in the meeting house; as with any venue, there will be a fee to use the space. There is a notable meeting house in center city Philadelphia (historic district):
4th & Arch Street: http://www.archstreetfriends.org/
Alternatively, if you are getting married in PA, you can obtain a Quaker Marriage (or Self-Marrying) marriage license from city hall (Orphan's Court: http://secureprod.phila.gov/wills/marriagelicense.aspx). It costs $90 (vs. $80 for a regular one), and it permits you to marry yourselves, as the Quakers do.
My husband and I did this and we chose one of our more eloquent friends to lead our ceremony. That gives you a little more freedom in choosing a location, so you could get married in a local park or even a decorated back yard!
More info about this option: http://malfiwisdom.blogspot.com/2009/06/eliminating-middle-man.html
We have shot some really spectacular small weddings at the Physick House in Philadelphia. The outdoor garden / patio areas are spectacular. Since your head count is small, it mau be a great choice for you.
I second rmalfi's suggestion of the "Self Uniting License". It's great. Lots of our couples choose this option. In fact, we had one that did their entire ceremony on the grounds of the Waterworks with this approach.
In our gallery, #59 is from the Physick House.
Additionally, you may want to consider the Valley Green Inn (on the Wissahickon Trail). It makes for a great outdoor ceremony. Sample photo #99 is from the Valley Green Inn.
If you decide that you really want a "chapel feel", you can always check out the Hammorton Chapel, in Kennett Square. It's not that close to Philadelphia, but it does have some great advantages. It's very flexible, not huge in size, and puts you with reach of Longwood Gardens for pictures.
If you would like to see any of the samples, just visit:
sugarloaf in Jenkintown area on Church Rd.
Click on each tool to see a sample.