Staff in hotels will appreciate a tip - $1 to $5 is fine depending on what service has been provided and for how long.

10% is welcome in resturants but there is not really a local tipping culture so you can get away with less or none if you want to (as salaries a low it is kinder to tip)

Uniformed car park guards which you will find in shopping centre and supermarket car parks are in salaried jobs but they will be very happy to receive a dollar or five rand if you feel like helping them out.  They will do lots of greetings and confirming your car is safe and hand signals to help you reverse out in the hope you may tip them but again it is not common for locals to do so.

If you park on the street you may find casual car guards popping up and offering to guard your car.  There is no need to accept their services or tip them when you return.  Depends really if you feel the guy is helpful and serious and not just pan handling.  Other guys may appear with a parking disc for your car in towns where this is needed.  These cost a dollar and as you may get a fine without one it is probably worth getting one.  They will also guard your car for the same price - ie the 10 cents profit they make on the disk! 

In Harare official parking agents will give you a parking ticket for a $ an hour.   

You may find people asking for money at traffic lights - normally children and blind adults being let by a child.  If you feel sympathetic towards their plight then any coins you may have in your possesion would be very welcome or a dollar even more so.  The average income in Zim is less than $2 a day so you can see how a small donation can make a difference.

On meeting anyone in a shop or hotel they will say 'Hello, how are you?.  It is polite and expected to say 'fine and how are you?' in reply before getting down to business.   If you don't you will find they reply 'fine' anyway as they assume you must have forgotten to ask them as not doing so is so unusual and rude they can't imagine it was deliberate.  This is a carry over from greetings in the local languages.