Weather

may is normally glorious weather in the day, can still be a little chilly at night for sitting outside but not cold enough to need a fire/heating inside.

hailstones and a lot of rain here today

we spent a few days in Porto last May , which is a bit further north and supposedly cooler, to us poor brits used to the wet damp uk, it was fabulous, it was low 70s all the time , maybe we picked a good week and was extra lucky, but it was glorious..
I feel the cold badly, and didnt wear anything thicker than a tee shirt the whole time..

Don't really know the Coimbra area at that time of the year but I'd say it could go either way. It'll be either glorious and hot but cool in the evenings or cool and raining.

If you wondering about clothes, take "layers" - stuff you can put on and take off as necessary - an umbrella might be useful too but I'd leave your wellies at home!