The Hyatt at Penn's Landing is at Spruce and Columbus Blvd (nee Delaware Ave.)

It's basically at the lower right corner of this map of the historic district:

http://www.nps.gov/inde/map.jpg

I'd say everything in the historic district is within walking distance.

The corner of Independence Hall is about 6 blocks away, maybe 15-20 minutes walking, but other stuff is closer and you'd probably walk between stuff in the park anyway.

Note this map is a little out of date - it doesn't show the new Vistor Center or the Constitution Center, which are both on the north end of the mall.

Penn's Landing is a turnaround point for many of the busses that run down Chestnut/Walnut, so there's good transit access there, but it's really not far to walk.

If you're arriving at the Greyhound terminal, it's off the map to the northwest. That would be a long hike with any kind of luggage, so I'd recommend a cab. You could take a bus if you're willing to walk a couple of blocks, but I imagine the cab fare will be around $5-$10 and well worth the money.

If you have the time, I recommend getting out of the historic area and seeing some other sights in Philadelphia. It's a great old city with a lot going for it.