Critique my 5 Night Portugal Itinerary

We did a tour with Vantage that included three nights in Lisbon and a 7 day Douro River Cruise. It was fantastic. Portugal is great.

1) If you haven't booked with airbnb, consider the InPatio Guesthouse a fantastic B&B in great location in Porto. Cost about 95 Euros per night. The best B&B ever.

2) You can walk around Porto on your own and see most of it in one day. Buy a Rick Steves guide to assist. Also, if you have time take a short river cruise.

3) Your time is Lisbon is somewhat short, but you can do Lisbon on your own. Rick Steves guide has three walking tours if you can handle that. If you don't want that much walking, suggest taking a city tour. Don't miss St. George's Castle at the top of the hill of the old city.

4) Sintra is good, we took a tour there. Not sure how convenient it is to take the train.

I think the plan is as good as the time you have available, so go with it. There is only so much "allocation" you can do with only 5 days and it seems like you spread the time among your two priority cities plus a short side trip. The only other alternatives I could imagine would require using one base and skipping the other altogether (meaning you'd either spend all your time in Porto plus day trips from Porto OR all your time in Lisbon plus any short day trips from Lisbon - that's probably what I would pick and save the second city and surroundings for another visit). The flight between the two cities is a smart way to save time, so two thumbs up for that. Given that you live in the UK, I think picking any option is very low risk since you can (I assume) come back easily anytime.

Thanks for the helpful replies!

I think I'll check out RS walking tours and make a list of things to see to maximize my time in Porto/Lisbon.

I do want to do a river cruise in Porto, just need to find/book one now.

I think there's a train that goes from Lisbon to Sintra for just 4 or 5 euros, but am open to using a tour company and being driven there.

There is a train and it's easy peasy (I'm not sure why the first poster made that comment - he must not have used the train)
http://lisbon-portugal-guide.com/Baixa-Lisbon/Rossio-Train-Station-Lisbon.html

Comparing train travel times with plane travel times is like comparing apples and oranges. With planes you have to check in at least an hour before your scheduled flight and if you have checked baggage, this adds extra time. Also, where are you staying at both ends? Travel time to and from airports is often longer than that to the train stations and taxi costs mount up. Also, by train, you see the country, not just clouds.
Don't try to see everything in one trip.
We recently spent 3 nights / 2 days in Lisbon before flying to Azores and only got a feel for the city, spending most of it in Alfama with a mid-morning trip to Belem. Get a Viva Viagem card for E6 for 24 hours if you plan to use trams & buses, but you need to go to a metro station.
Have fun.