Part of why you are confused is your terminology. Usually, "train" refers to something taking you to a different city, or at least to a suburb. For travel within the city, you will be taking the metro (in Paris) and the tube or underground (in London).
I can hear the Forum regulars screaming: "That's not the whole story! What about the RER (for Paris) and the Overground (for London), not to mention all kinds of other things on rails that are in these cities?" They're right. But, none of these things are referred to, in usual usage, as a "train." That's why a "rail pass" won't work - you are mostly using local transit not covered by a rail pass (like Eurail) that is designed to cover longer distance trains.
For London, there are multiple zones, and you can cover your travel with them with an Oyster Card. Most day trips from London fall outside the London zones, so you will not be using your Oyster, but rather buying rail or bus tickets separately. However, some places like Greenwich, Kew Gardens, and Hampton Court Palace are within the London travel zones.
For Paris, the city itself is zones 1 and 2; CDG airport is zone 6. Like in London, some day trips (like Versailles) are within the Paris transit system zones, while others (Giverny or Chartres) are not.
If you tell us how long you are in London and in Paris, and which days of the week you are in Paris, we can help with what would be your best deal for local transit in those cities. Days of the week matter for Paris because the one week pass there is only for Monday through Sunday; the London one week pass can be for any seven consecutive days.
For the day trips, it depends on which specific ones you had in mind. Some will have discounts for advance booking, some will not.
Again, you're not wrong - it IS confusing.
