Subject: GPS usage
Don't know what GPS model you have but I presume you'll have to buy a map for it covering Paris streets? A lot of money for a relatively short visit.

In any event, I find GPS a bit difficult to use in cities ... reception comes and goes due to tall buildings. Probably less of a problem in Europe than in the US since cities there tend to be a lower-scaled.

That said, I find the most useful piece of technology in strange cities is a $4.95 compass. So many directions given say, to the east, etc ... which helps little if one doesn't know where east is. And a compass works in the Metro, on buses and in the hotel room as well as on the streets.