Hmmmm.... It's hard to say if I could recommend a full half day in Downtown Disney right now due to how much is under construction. That's not to say that there isn't a lot to do right now there, but it's a bit less than usual and Downtown Disney is a much smaller thing vs Disney Springs at WDW. There is plenty of retail therapy and eats to try. If you aren't staying at or have visited the Grand Cal, you should also take a detour to check out the lobby and other things there. We actually love eating at the GCH Craftsman quick service even if we aren't staying at the Grand Cal when we have time. Breakfast with characters at Storytellers Cafe is a great option if you don't have a park ticket but want an awesome experience.
If you are a hotel/resort fan like us, you might also consider doing a tour of the new
Disneyland Tower DVC property while you are visiting to kill some time (if you are visiting after it opens). We love DVC tours and as long as you don't mind asking, you and the family could walk away with a fair amount of free stuff. Word of caution... DVC can be an expensive hobby