Shopping at the towers is great and I have found items which I would not have gotten anywhere else. Eating at the towers is great too, though more exclusive and more expensive, unless you just visit one of the food stalls.
Getting a ticket to walk the bridge is A MUST!! I always reserve mine online, so I don't have to stand in line and run the risk of the tickets being sold out. Last visit I timed my visit to the bridge so I can do "sunset" from the bridge and got lucky!
And I have recently done the visit to the Viewing Deck on the 86th Floor, which is truly awesome!
I also do enjoy walking in the park in front of the towers and enjoy the ever changing views from there during the daytime. Though recently there was a lot of construction going on and parts of the park were closed. Night time is of course also one of my favourites and the water fountains in front of the towers offer some great photo opportunities.