C'mon - if you're staying in downtown Philly and have never viewed the place where the U.S. Constitution and Declaration of Independence were written, you're missing out.
Tours run from 9a to 5p and cost nothing. But, tickets are assigned by time, so you can't just waltz through whenever you please. Still, it's an easy and convenient tour...and it's free!!
There is also an option to reserve tickets in advance for $1.50.