Not specifically metalworking, but it's interesting to walk around free in the Philadelphia Navy Yard, which is 4 to 6 miles northeast of the airport, at the south end of Broad St. Ships in drydock, decommissioning, etc at "NAVSEA Inactive Ships On-site Maintenance" site, eg USS Forrestal and USS John F. Kennedy aircraft carriers at pier 4 a couple blocks west of Broad St.
A few blocks east of downtown at Penn's Landing, if you have a free day get a combined waterfront ticket ($$) and tour USS Olympia (the pre-1900-vintage machine shop is interesting), USS New Jersey, USS Becuna, Independence Seaport Museum, etc.
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