Steves says that locales like concert halls have a casual dress code during high tourist seasons, but you should bring items like slacks and a collared shirt for dressier events.
Shorts are also inappropriate for certain locations. Steves writes, "Some churches, mostly in southern Europe, have modest-dress requirements: no shorts or bare shoulders."
Shorts don't have to be totally out of the question — it's just important to know when to wear them. Avoid athletic or cargo shorts since those are uncommon in European fashion.