There are decent public toilets at Piata Victoriei metro station (free), and at Gara de Nord (for which you have to pay, 1 leu). You will find free portaloos around the city, but these should be used only by the very desperate. Otherwise, find the nearest McDonald’s and make use of their facilities. (Note, however, that entrance to some McDonald’s toilets in Bucharest is protected with a four-digit code, which you will get when you buy something).