Hi,
Although one needs to be circumspect in any large city, during daylight hours I believe you will have no trouble in and around Sandton on foot and alone, including the parks.
Closest cache to you hotel is one of mine "Which Rock?" and is an easy walk to a nearby park - about 1km from the Wanderers club. Another very nearby one is a mystery cache "Logical 1" which is also about 1km away in the suburb of Birdhaven. You've already been told about Buonarotti (which I haven't done yet). Another of mine is at the Inanda Club between Sandton Central and Wanderers called "Mink and Manure".
I'd be happy to meet up with on Saturday and help with some transport - I live not far from the Wanderers. If you are interested then contact me directly.
Regards
Derek