We will only be in Paris one day, so the Ticket Mobilis seems like the best option. Will this cover us for metro, RER, and busses?
Ticket Mobilis is good for Paris Metro, RER trains and buses.
Keep in mind Ticket Mobilis is not valid from CDG Airport, nor OrlyVal trains. UPDATE 2023: Ticket Mobilis now covers CDG Airport on RER B, Roissybus and Orly Airport by Orlybus & Tram 7 (still not Orlyval though).
If you need a Paris day pass valid for CDG airport travel, use Navigo Day Pass 1-5 zone or 1-4 zone for Orly OR use Ticket Mobilis, using Tram 7 or Orlybus (not OrlyVal train, which only accepts Paris Visite 1-5 zone tickets).