I am going to be in Lisbon for 6 nights. Planning day trip to Sintra,and Evora/Coimbra. Is it possible to do a day trip to Porto, or is the distance too far? What about a private guide/driver?
Possible...yes. Distance too far...for me as a day trip...yes. I would not bother for the value of your time. There are loads of options in Lisbon or within an hour or so to occupy your 6 nights and are truly worth it for sightseeing. Now if you could catch an early train and stay overnight then it could be worth your time. Even still, I would not. Others here could give great opinions so weigh out your decision after seeing some replies. Good luck!
You can check out these two companies for private guide and driver. Porto is about a 3 hour drive from Lisbon, so it's a very long day. I think Porto is worthy of more time personally.
Moor Viagens
http://portugaltransfers.pt/
Amiroad
http://www.amiroad.pt/amiroad--luxury-transports--en.html
You can take a train from Rossio station to Sintra and then buy a one day bus pass to get around Sintra. This is quite straightforward.
We used Moor Viagens for transfer from Lisbon to Porto and stopped on the way at Obidos and Coimbra. We stayed in Porto for 4 nights however.
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Driving doesn't sound faster than the direct train from Lisbon to Porto, but could depend on your proximity to the train station. Either method takes about 3 hours one way.