I wouldn't do no repeats, with the exception of spells. I mean, that poor sad little fighter who just swung his sword at the ogre and can't do it again with his second action, just staring at the spellcaster lobbing two spells.
What I would do is say if you cast a spell, the second action has to be a spell that uses a lower spell slot. So if you cast a fireball with your first action, the second has to be 2nd level or lower. If your first spell is a cantrip, you blew it. Highest spell has to go first. Though I would make an exception for cantrips. Two cantrips seems fine.