Marbella is a wonderful place to visit but it is important to know that there are two sides to Marbella and they sit at 4 miles apart.
On one hand, you have the glossy harbour housing the billionaires' boats which make for an interesting eye opening stroll along the edge of the water. The high end designer shops complement this aesthetic alongside some people watching, al fresco cafes. This is Marbella's Puerto Banús.
On the other hand you have the stunningly quaint and pretty historic town. And it should be visited. Beautiful court yards hosting orange trees and ancient chapels break up the tight alleyways that take you on a journey passing hole in the wall restaurants, small boutiques, old houses and plant pots galore. You could spend a wonderful lunchtime or enjoy early eve tapas while listening to local musician buskers while sheltering from the sun.
For the more active, a wonderful way to see both sides of Marbella, is to hire a bike from the sea front in front of the old town and cycle the 4 miles there, all the way along the prom. Its a wonderful cycle ride with the odd restaurant hanging over the beach as you journey along passing some beautiful architected homes amongst the palm trees.