According to the Turismo do Centro Post at Portagem in Coimbra, Cafe Cristal, next to Coimbra-A Station, which used to offer a left-luggage service for day visitors to Coimbra has closed.
However, according to the same office, apparently, on weekdays only, there is a company, Go Walks, that has lockers as follows:
Mobile: (00351) 910 163 118
They are apparently situated in the centre of Coimbra by the river, upstairs from
Largo Portagem 10