A valve adjustment is a good idea and a new valve cover GASKET is always a good idea when you have the valve cover off cuz they are only like 10 bucks. If the plug they are talking about is a cam seal, then the rest of the money is probably to replace that. The only other plug type thing I culd think of is maybe the little rubber things that go under the nuts that hold the valve cover on. I wouldn't let them replace the valve cover unless it has some kind of dent or crack where the gasket is which would make it seal improperly. Which I don't see happening on a car with that few miles. A completely new valve cover is not needed and would probably cost 250 by itself from a Honda dealer let alone all the other things they want to do. Get a detailed description of what they want to do and then post it here.