Should we rent the car before arriving? We are only using it for one day. Any suggestions or tips would be helpful. Thanks,
You will probably pay less booking in advance and from the U.S. than showing up at the counter. Plus for something so critical, you don't want to be caught without a car because they did not have availability. I think Autoeurope has a three day minimum.
It might be simpler and perhaps cheaper to get it arranged over the internet before you leave, however the concierge at your hotel will be able to direct you to convenient rental locations. Be forewarned that Portugese drivers have a well-deserved reputation for some of the worst driving in Europe.