We are staying at the Trianon Versailles for 2 days before our tour begins and want to spend one of those days (Saturday) in Paris visiting the Eiffel Tower, etc. What's the best way to get there? Are the hop on hop off busses a good idea (one of us has some what limited walking ability)? What kinds of admission tickets can we buy in advance.
Versailles is in zone 4. The closest station to the Chateau de Versailles is the Versailles-Rive Gauche RER station. The RER C line goes directly from there to Champ de Mars-Tour Eiffel which is the closest station to the Eiffel Tower. Your hotel is closer to the Versailles-Rive Droite SNCF station so you'll want to arrive there when you return. Buy one-day, zone 1-4 Mobilis passes (€10.55) at either station. The passes will cover your rides to Paris and back to Versailles and all the rides you take on the Metro, RER, SNCF trains, or bus while you're in Paris.
Ithink the hoho buses can be a fun and easy way to get a nice overview of a city. A few things to think about though, for me the fun is sitting up top and being able to enjoy the views of the neihborhoods as you pass through them, so if the weather is rainy that is not an option ( they are open top) . Also I like L'Open Tour buses, they are the bright lime green and yellow ones. They have more routes and more stops, and they sell a two day pass that is only a few euros more then a one day pass.
The buses stop at all main tourist sites, and of course that includes the Eiffel Tower. You can pay for your pass when you board, no prepayment needed. The closest RER line you will come in from Versailles in has stops right by ET. If mobility is an issue do not use metro, many stations have longish walks and many stairs up and down. Take the public buses instead( this assumes you choose not to take a HOHO ) they are much easier then metro for elderly or mobility impaired persons.
If you decide to buy tickets for a HOHO bus tour and get on after you've visited the Eiffel Tower, then ignore my recommendation to buy Mobilis passes. Buy single tickets (€3.25) to get from Versailles to the Eiffel Tower and single tickets to get back to Versailles at the end of the day. For a zoomable map of the Metro and RER system that includes SNCF train routes, go here: http://tinyurl.com/6xmvms4.