Leveling-smoothing an exterior cement patio
The only good ways to get a truly smooth surface at this point is to 1) cover the slab with tile, or 2) grind it smooth. 

You can install porcelain or quarry tile for a truly finished surface. Obviously there's some expense and effort involved with this option.

On several occasions I've seen businesses bring in companies to grind their concrete to a polish finish. It's visually attractive and very smooth.