Have you heard about the Egyptian Lighthouse (one of the seven wonders of the ancient worlds) that was found in the sea? Well, people are proposing lifting the 70 ton sections of the light house with balloons. Can a balloon do this? What special aspect of the balloon will lift 70 ton bricks? What kind of balloon would be used?

I would guess that they intend to you balloons in the water. The blocks are at the bottom of the sea, so they must be lifted up to a boat. A balloon experiences an upward net force that is equal to the difference between the upward buoyant force on it and its downward weight. If the balloon displaces air, then the buoyant force on it is rather small and it would have to be extraordinarily big to displace enough air to lift a 70 ton block. But if it displaces water, then the buoyant force on it is much greater. To displace 70 tons, it would only have to displace about 65 cubic meters of water. That's not hard at all. The balloon could be made of heavy reinforced canvas and still work just fine underwater.

I understand how dimples on a golf ball reduce pressure drag, but wouldn't they also increase viscous drag? (i.e. a rougher surface experiences more "friction" from the air).

Yes, the dimpled golf ball probably does experience more viscous drag than a smooth golf ball. But viscous drag is only a small fraction of the total drag on the ball. Pressure drag is much more important (and much larger).

My big square truck creates a lot of turbulence when it moves. Does my roof rack (a factory-installed one, close to the roof) actually improve aerodynamics, like fuzz on a tennis ball? (Also, what about the air dam at the back end?)

I'm sure that modern car designers consider aerodynamics when building a car or truck. They do structure the trailing edge of the car to minimize its turbulent wake. But I doubt that a roof rack helps much. It's probably too tall for the boundary layer on the car and extends into the free flowing stream beyond. As a result, it probably experiences its own pressure drag. The "fuzz" that trips the boundary layer has to be no taller than the boundary layer itself, otherwise it causes turbulence in the main airstream rather than preventing it. The same goes for the air dam.

How does fuzz on a tennis ball make it fly faster? It seems counterintuitive to me.

Yes, it is counterintuitive. The reason for the fuzz is that a swirling layer of air close to the ball makes a good buffer between the ball and the main airstream. As a result, the main airstream flows most of the way around the ball before it breaks away as a turbulent wake. Without the swirling layer of air on the ball's surface, the main airstream encounters backward-flowing air near the ball's surface and breaks away from the ball early, leaving a huge turbulent wake.

Does the decreased density of the air in Denver make it easier to achieve turbulent flow at the boundary layer of a baseball and therefore make the ball fly farther?

Whew, this is a toughy. The air in Denver is less dense, so it tends to respond better to viscous forces. On that account, it would tend to be less turbulent. But it is also "thinner" and less viscous, so it would tend to be more turbulent. I think that those two effects essentially cancel, so that the ball experiences the same degree of turbulence at any altitude. However, the air in Denver has less pressure, so it exerts smaller forces on the ball than air at sea level. Thus, although the flow properties aren't affected by the increased altitude, the pressures involved are. The ball should certainly carry farther in Denver than at sea level. Imagine playing on the moon, where there's no air at all. The ball wouldn't experience any drag at all!