Two very good choices, we've done both, on numerous trips.
Fish Friday in Gouyave - its up the coast from St Georges and you'll need a taxi. Don't try and drive it yourselves, twisty roads, narrow in places and dark. You'll want to arrive early, about 6:00 or 7:00 pm and take a walk through the vendors BEFORE you start sampling. (My rookie mistake - hit the first two vendors I saw, it was GREAT, but I was full and there is sometimes better choices around the corner. Very safe, great local vibe, often live music a fun evening and you can see and sample the event in a couple of hours. It gets pretty busy later in the evening, and if you are going to market Saturday...
Market day Saturday - busy BUSY at times, but wow, produce and spices everywhere, you can barter a little but prices are pretty reasonable to begin with. You should take a taxi or local bus to the Carenage and walk through the tunnel to get to the market. A taxi right to the market will be a lot slower, traffic is slammed around the market area. You'll find some nice souvenirs, we buy our fresh fruit and veg there but we have been staying in a cottage. The fish market is close by and worth a look if you are there early. Lots of neat places to pause and get a cold drink and watch the city go by. Near the market is the cruise ship "mall" if there are ships in port, expect bigger crowds. Typical ship souvenirs and prices will be higher than in town.
A very good local guide we've used several times is Mandoo. You'll find his site here: http://www.grenadatours.com/ we've used his service to take us to Gouyave as well. See if some new friends from the AI want to join you Friday night, split the cost. He has set itineraries, but can and will tailor a trip to suit.
Have fun, please do explore the real Grenada and not just the AI world of Sandals.
Expect people to wish you "Good morning" or "good afternoon" everywhere - the Greandian people are among the friendliest and most genuine in the Caribbean.
Edited: 7:48 am, July 28, 2013