hardly ever violence here. small crimes like pickpockets etc. if in a house, get a 24/7 security for about 400 - 500 cedis a month (e.g. G4S). dont leave your car and house unlocked, if in crowd carry purse/wallet in your front pocket. but that's about it.
oh yes, one more thing. dont brag and dont show off. everybody knows you have more than the locals. there is no point in increasing temptation.
otherwise, everything safe (in the city), also at night no problem going around.
try not to drive outside the city at night, plan your journeys so that you always drive during the day.
ghanaians are peaceful and life is very forgiving.