Welcome to Fluther.
I highly recommend a guided canal boat tour. The tour operators are real pros at presenting the city, the canals are ubiquitous through the town, and you’ll sit in a comfortable seat and listen to an entertaining and humorous – and interesting – presentation of the city, giving you many more ideas of “what to do next”.
And say hello to the folks at Café Pacífico in the Red Light District for me (enjoy the lemon sorbet there, too).