Would probably stay clear of shibuya/Shinjuku (it's easy to get lost. Good if you have spare time, not good if you're trying to find the right subway/track).

Also depends what you want to do? You can have a 10min meal at a Japanese fastfood place i.e. ramen place/gyudon(sukiya/matsuya/yoshinoya/...) with more time for exploring/buying snacks at Don quijote, or spend 30-60 min on a good restaurant

It's quite common to see people with carryon roaming around (or stick it in a storage locker, but that requires suica or coins/cash. Recent iphones can create virtual suica cards)