You can make bingo cards for both (words for the eldest/pictures for the youngest) and they can mark things off as they see them. Have a small prize for a line and then a full house.
I find six year olds love trivia questions or maths questions so they can show off how clever they are. You could also include the three year old in this if they're that way inclined.
Sticker books/activities are great for this too. Does she have a tray for her lap so she can draw etc? Things like making a paper chain can keep them busy for hours and relatively easy if you cut the paper ahead of time and use pritt stick. It's also fun to get it out full length each time you stop to see how long it's getting.
In terms of chatty games:
What can you use X for? For example, what can you use a thimble for.....a bath for an ant, a cup for fairy, a hat for a tiny man, a very small snack pot etc.
Word at a time/line at a time story - tell a story, everyone takes it in turn to contribute a word/free words/line of the story.
Yes/No game. You have to try and make her day yes/no she has to avoid saying it and vice versa.
The alphabet game go through the alphabet thinking of an animal/person you know/food etc for each letter.