In Plato's Philebus Socrates summarizes two views he is about to discuss with his interlocutor Portarchus:
Philebus says that the good for all animate beings consists in
enjoyment, pleasure, delight, and whatever can be classed as consonant
therewith, whereas our contention is that the good is not that, but
thought, intelligence, memory, and things akin to these, right opinion
and true reasoning.
[Plato, Philebus, 11b]
From the above we learn that according to Philebus, pleasure is the ultimate or intrinsic good whereas according to Socrates - knowledge is the ultimate or intrinsic good. Both views display monism about value.
The view held by Philebus falls under the heading of Hedonist theories; and that expressed by Socrates - under the heading of objective goodness theories.
Usually objective-goodness theories display pluralism about value, and thus they can in principle consider knowledge as well as pleasure and other things as intrinsic goods (as opposed to instrumental goods). Derek Parfit, for example, mentioned "the development of one's abilities, knowledge, and the awareness of true beauty" as intrinsic goods (See Parfit, Reasons and Persons, 1984). Such theories are occasionally referred to in contemporary literature by the name ‘objective-list theories’.
(1) Within the framework of value-theory, to which your question falls, the distinction between intrinsic and instrumental value can be clarified as follows: Things that are intrinsically good or bad are things which are good or bad in and of themselves; things that are instrumentally good or bad are those we would identify as good or bad only insofar as they contribute to something else.
(2) A similar conceptual distinction to that between ‘intrinsic value’ and ‘instrumental value’ can be found in David Hume’s Enquiry Concerning the Principles of Morals in which Hume writes:
Ask a man why he uses exercise; he will answer, because he desires to
keep his health. If you then enquire, why he desires health, he will
readily reply, because sickness is painful. If you push your enquiries
farther, and desire a reason why he hates pain, it is impossible he
can ever give any. This is an ultimate end, and is never referred to
any other object. Perhaps to your second question, why he desires
health, he may also reply, that it is necessary for the exercise of
his calling. If you ask, why he is anxious on that head, he will
answer, because he desires to get money. If you demand - Why? It is
the instrument of pleasure, says he. And beyond this it is an
absurdity to ask for a reason. It is impossible there can be a
progress in infinitum; and that one thing can always be a reason why
another is desired. Something must be desirable on its own account.
[David Hume, Enquiry Concerning the Principles of Morals Appendix 1, §18].
(3) It is intuitive to think of pleasure as an intrinsic good, and on economic wealth for example as rather an instrumental good. As to knowledge - it is less direct to see how it differs from economic wealth, but one who contends that knowledge is an intrinsic good can follow Socrates' line of thought, namely consider knowledge to be a virtue.