Guelmim (also spelled Goulimine or Goulimime) sits at the edge of the Sahara in southern Morocco, historically the meeting point between nomadic and settled populations. This is the gateway to the desert south, though you are still far from the dunes — for a real Saharan experience, head to Merzouga.
The main reason to come is Saturday market day, when people from across the region descend on the city. This is your chance to see real nomads in their indigo-dyed garments — the "Blue Men" of the desert. In earlier days the dye would tint their skin, hence the name. Watch out for staged tourist shows where locals bring visitors to tents outside town for a theatrical "Blue Men" experience; the Saturday market is the real thing. If you can time your trip to coincide with the regional moussem in June, the spectacle is even greater.
Guelmim is also a good base for visiting nearby Sidi Ifni, a former Spanish enclave that still has a distinctly colonial feel to it.