Go to The Walk. Great restaurants (I recommend Paul to everyone), outdoor dining (the weather is great right now), and good view of the ocean and the Palm island.
Careful, there are two "Walks" in Marina. One is called "Marina Walk" which has an amazing view of Marina's skyline and yachts, the other one, that has been mentioned here several times is the "Jumeirah Beach Walk (JBR Walk)", also in Marina.
Both are good, I recommend the JBR Walk at day time and the Marina Walk at night time.