Marrakesh. Weather good, safety good, super interesting, spa days possible, day trips for hiking to the Atlas Mountains (Google Le Toubkal to see a good place), food stupendous. If you are into shopping, the souks are fabulous, and if not, just spend your days in places like Majorelle Gardens and you will get relaxation in spades. I think BA flies there from Gatwick (just re-started or will just re-start this route, IIRC).
If you don't want to get that adventurous then I would second Lisbon.