Paladin Smite: best solution would be to have a popup that asks whether you want to apply a smite (along with a box to check for the level of the spell slot to use). This is what Solasta does, and in my opinion it just works, and does not create too much hassle.
However, an alternative approach would be to have it be similar to the opportunity attack button. You toggle it when you are attacking an enemy that you would like to smite. The key here is that it needs to use the spell slot ONLY on a hit. I personally would find this implementation annoying, since I will inevitably forget to toggle it off and waste it on a half dead goblin. There would also need to be some mechanism for indicating what level of spell slot you want to smite with.
IMO rogue sneak attack should work similarly, toggle it on and then use it up when a hit occurs.
