If you're an airplane geek, the PHL Airport Marriott overlooks the entire PHL runway setup and gates (from the correct side, although they have windows at every elevator you can look out.) The Ren also isn't bad for watching takeoffs if you have the right view, but it's more hit/miss.

IIRC, but it's been a few years, the Ren Baltimore Inner Harbor has some good views, as does the JW in Washington DC.

Hard to beat the Fallsview thought!^