To me, when you say you will ready an attack, you are readying an attack action. Thus, I would allow a swing of a weapon, an unarmed attack, a trip, a grapple, a charge, etc as they are all attack actions, just different forms of attacks.
Spider said:Let's say I want to ready a counterspell. If I can't counterspell a spell he's casting (I don't have Dispel Magic, or the same spell), can I cast something else?