A half day should be sufficient to explore Strasbourg. Going there for a day-trip via TGV from Paris in August, in at 12 noon and out by 8 pm, primarily just to get out of Paris in the middle of a 5 night stay and it’ll be our 4th time in Strasbourg and each time we’ve never spent more than a full day.
A day stroll in Colmar and Riquewihr would be fun for your 19 year old with the fairy-tale-ish look and structures.