When visiting Mombasa be aware this is a poor country! If you show off your gold, diamonds, money and phone you may become a target for local thieves. Ladies should not walk around with handbags, ruck sack's etc. Carry small amounts of money around with you, maybe in your bra, a money belt, or deep in your pockets if they are tight. Maybe also carry two separate stashes; if you are unlucky enough to be robbed, give them the small amount and hope that they run off.
Mombasa is generally safe and most parts of the coast are safe apart from Lamu, what to avoid is going to Lamu near the Somali border. But its just a matter of time before the area becomes secure.