I plan on driving from Duoro Valley to Lisbon and stopping in Coimbra along the way. Walk around have lunch etc.
#1 - I am concerned that I will be in a rental car and I can't leave my suitcases in the car. Any idea on safety? Where I can possibly leave them, like a locker or possibly hotel would hold them or maybe they are safe in a parking garage? All thoughts welcomed!

#2 - any suggestions for an easy place to have lunch? Preferably outside.
Thank you!

We were lucky. We fell upon a small parking lot to the right of the central bridge which was guarded, perhaps unofficially. We gave a tip when we left. I recall payment being confusing.

This is the bridge

https://flic.kr/p/7NuJeY

Mercado Municipal and Il Tartufo (Italian) are good options for lunch. The Mercado has several places to choose from.

This is the car parking that the above refers too. https://goo.gl/maps/FT2NgEMeZBvwSaSA6
Go to this website. Add your destination and choose a parking. It will give you details. even websites for the parking. https://www.parkopedia.pt/
Most underground parking will have cameras. Personally I would not worry about suit cases in a car too much. Done it many times, with no issues.

For lunch, in the lower city:

https://flic.kr/p/2gxvfdZ

Thank you all so much! We leave Sunday for Porto! You have been such a huge help I will look for that bridge, parking and lunch suggestion! Thank you any thoughts on Porto for 2 days.

With only two days in Porto, you might consider the free (tipping suggested) morning walking tour with Porto Walkers to see highlights. They also do an afternoon tour of a few port houses with nice tastings for a reasonable cost. And when that concludes, you are near to the food hall in Gaia that offers several casual options with good food. The Serralves museum is definitely worth a couple hours.