Let us know how you deal with your baggage. Marriott is attached to airport, perhaps they offer this service.
Trains run every 25 minutes- it’s a short ride to 30th street station main terminal. The train stops at penn campus.
Your daughter and boyfriend will be your tour guides, so most likely they will select area and restaurant.
I can highly recommend uber vs taxis in Philly. Had excellent experience taking $5 uber from art musuem to sabrinas- on callowhil ( daughter says may be wait for Sunday brunch- popular casual)
Might be nice to dine near one of parks, rittenhouse park,surrounded by restaruants, PARC open air overlooks park. If not too humid might be option- check yelp, they have always been spot on.
Tulula’s next to Washington square park ( my fav) and a short block or two to historic area. With 8 hour layover, gives you opportunity to see a bit of Philly. If there is a line for liberty bell, just walk up to plexiglass window which gives you photo op for bell, without security checkin and lines.
Another way to see Philly, especially the areas ( parkway) where pope will be in September- is the phlash - continuous trolley cars which loop around city, stopping at main areas- reading termino( food emporium) love park, museums, rodin museum, art museum, penns landing, even going out to fairmount park, site of Philly centennial exhibition and please touch musuem.
Let me know what you find out about luggage- enjoy