Looking for a couple of recommendations from the vets here.

I have a 2013 giant escape 0 that I use for commuting to work (6.5 miles each way with a 450 foot elevation change) as well as general errands. Currently I am just using a backpack for cargo but am looking to upgrade to a rack or something better for use when I have heavier / bulky stuff to transport.

Also, I'm thinking of getting more traditional road tires. My bike's tires are very "hybrid," fairly thin but knobby. Any thoughts or suggestions? My ride is entirely on paved surfaces.

Smooth "tread" tires for the street..

Front Basket [Wald?] to put your back pack into, or rear rack + basket.

Gets it off your Back.

Blackburn racks are excellent. Extremely tough but light.

For tyres, yes, get treadless "slicks" or almost treadless semi-slicks. Different tyres give a different mixture of speed, wet and dry grip, pucture protection, etc. If you take a look at Schwalbe's site they give relative ratings for all these factors for many of their tyres - but other people like Conti and Kenda make good tyres too. Just be aware tyres vary in their mixture of properties - eg a Marathon Supreme is good at everything including speed but costs a lot, and a Marathon Plus is cheaper, the closest thing to absolutely puncture proof, but a real boat-anchor. The re-released Avocet Cross should be a very good all-rounder.

You might also think about adding Ergon Grips with bar ends, and switching to Koolstop pink brake pads if you ride in the wet.

Rear rack is the only way to go, I keep a motorcycle cargo net on it and can haul almost anything.

I have Schwalbe Marathon + on one bike and the tires are bullet proof although the ride is not the smoothest.
The other MTB has Serfas Drifters 2" x 26" and the ride is so comfy and it floats over everything, no flats on either tire after several hundred miles.
The best thing is to get rid of the knobby tires and getting the weight off your back is the next best thing.