Layover #1: 9 hours (11 am - 8 pm) just me, and I will likely be jetlagged. In a perfect world, I would go to Sintra, but with passport control, stowing my carry-on, transportation time and checking in again, I figure I really only have 5-6 hours.
Layover #2: 23 hours (starting at 5 pm) with a 21 year old daughter with many food allergies. No wheat, fish or nuts. She will want to visit the bars at night. The next day, we can tour until around 1:30 before heading to the airport. I'm guessing she will want to see Baixa, Chiado and Alfama. Could we see those three areas (and possibly Belem) in a morning?
I don't want to do the same things twice, which is why I was leaning toward Sintra. I could visit Belem by myself, or do a food tour, or??? Suggestions? I am willing to hire a guide if necessary.