Quite easy to spend a splendid day in Strasbourg! We were there for 10 days in 2017, and will be back for the whole summer this year.
I will echo what others have said: a stroll in La Petite France, a gander in and around the Cathedral (and maybe a trip to the top of the tower), and the Batorama boat cruise are all great options.
If you're looking for a spot to eat a leisurely lunch or dinner or sit back with a beverage, La Corde a Linge and Au Petit Bois Vert (or its 3 neighboring cafes) are great canal-side spots in La Petite France, and the various places encompassing la Place du Gayot offer a lively but tucked-away spot.
You can also get a fast and cheap train down to Colmar and have a great day there: covered market, Unterlinden museum, charming Alsatian architecture.