The easiest way to navigate is to realize that the convention center is a few blocks south of the official "downtown" point of Miami. (400 SE 2nd Avenue to be precise.) The downtown area's laid out on a (somewhat irregular) grid system so you can measure any numbered street by looking at its direction and number.

This whole area is generally safe, so walking is not a problem, but there's only one property less than a mile from the convention center - the Sheraton Biscayne Bay (about 1/2 a mile southeast of the convention center.) To get to restaurants and other stuff (including the tri-rail transit system) you'll have to go another few blocks north of the convention center.

If you're open to the idea of other chains, the Hyatt Regency is actually inside the center, so you will never have to set foot out of doors. It's decent - aging a bit but well-maintained and (for the most part) well-staffed.

This year, however, I stayed in the Clarion which is right next door. It was definitely shabbier (it is, after all, a Clarion) but the service was on par with the Marriott and the room was 1/3 the price. I have recommended the Clarion to two different people in the past few months and both came back saying they were pleasantly surprised.

By all means, AVOID the Dupont Plaza (the other close-by hotel) if it is in fact still standing. It's scheduled to be torn down and while I'm sad to see a historic property go, it was pretty much gone already.

There's not much really good food downtown. All the "good" restaurants are going to be in Coconut Grove or South Beach, which means a taxi or bus. If you're adventurous, the bus system is pretty easy to navigate and very clean.