Donna,
We did an all day private tour there in June with a great guy. He was about 27 and just as nice as can be. We spent the first 1/2 day in Sintra (actually ran over a 1/2 day) then spent the afternoon in Lisbon. There were 6 of us in his van. Our guide Josue (he said to call him Josh) had a lot more energy than we did and was going strong. I'm sure he thought we were a bunch of wimps but he was very understanding and let us modify things along the way. Olivia's daughter really wanted to shop so he took us to a fabulous shopping area in Lisbon which is 100% pedestrian walking and no cars around. If you want to use the company we used you can look them up on www.toursforyou.pt. We also ate some delicious pastries in Sintra. Josue took us to a very nice restaurant for lunch (not the cheapest place but it was very nice). He's receptive to do what you want to do.
Toursforyou originally wanted us to wire them $$ as a deposit but I said that this wasn't possible. I promised them that I was a person of my word and would not stand them up etc. so they let it slide without any deposit. If you decide to use them, be sure and mention me (or Maurice)as a reference and they'll probably let it slide for you too. Josh is the same guide that Maurice (med4bike) used a couple of weeks before us!

We paid 270 euros for the entire day (not per person).

I'd highly recommend him and his company. Judy and John also have him lined up for later this month.