Actually Mugabe ran the country pretty well in the first 15 years or so. Of course there were some things that he did that weren't exactly saintly (the Matabeleland massacres, accusing whites of espionage etc.).
But up until 1995 things were looking pretty good in Zimbabwe. The country had invested heavily in education and healthcare and had the largest middle class (proportionally) in Africa and one of the best educated populations.
Things started to go pear in the late 1990s for a variety of reasons.
So, have Mugabe (for whatever reason) persuaded to retire in 1995. Perhaps Sally, his Ghanaian first wife, who was by all accounts a positive influence on him, doesn't die in 1992. She keeps Bob on a decent path, and he stands down in 1995. He will, of course, be succeeded by another Zanu-PF strong man, but someone who may not be as rigid as Mugabe. The MDC still gets formed in 2000 or so, but the political climate is much less poisonous, and there is a peaceful transfer of power to the MDC in 2005, after a relatively open and free election. The country still goes through something of an economic crisis, but the demonisation of whites and total collapse of the economy doesn't happen.
Mugabe is still alive, but is something of an elder statesman. Not a Nelson mandela, but on the same level as Kenneth Kaunda, Jouquim Chissano, or Quett Masire, well-respected former African leaders.