We are planning on visitng Porto and Lisbon for a total of 5-6 days in September. Is that doable and would love to have suggestions for daytrips from both cities and what to do in both cities as well as what can we expect weather wise.

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Hop on Hop off is a good way to initially see each city. You might like to check out the pousadas also..Luxury at somewhat reasonable prices..There are special rates available and LHW has a glorious place, Pestana Palace worth a look see. Sept. will be quite warm, even hot.

How warm is warm, even hot?

For temps see weatherunderground.com.

For things to do in Lisbon see http://www.fodors.com/community/euro...ing-lisbon.cfm. Day trips to Belem and Sintra are possible, but you really don't have much time.

Take the train to Porto (see seat61.com ). You could day trip up the Duoro valley or to Braga.

I was in Lisbon at the beginning of October and it was quite hot - 85 to 90 degrees F. in the daytime.

5 to 6 days is not very much for both Lisbon and Porto. I suggest you stay in Lisbon and take some day trips to Sintra and possibly other places in the area and give yourself 3 days in Lisbon/Belem. Sintra deserves at least a day or a bit more.

From Porto, definitely do a Douro Valley trip. You can do the train, the boat, or a car/train combo. The scenery is spectacular.

From Lisbon, take the train to Sintra, and tour the palaces.

Only 5 or 6 days is not enough to see both cities, especially when a trip to Lisbon also implies a daytrip to Sintra. Nonetheless, you can still go to Porto from Lisbon and stay two days there, two days in Lisbon, and one day in Sintra. Here are a few good ideas for Porto http://www.lisbonlux.com/magazine/fr...reasons-to-go/ and a good itinerary for your 2 days in Lisbon http://www.lisbonlux.com/lisbon/itinerary.html