I'm trying to get a better ground to get into Continental Rationalism between Descartes, Spinoza, Leibniz, etc. I read on the Plato Encyclopedia of Philosophy that Plato and Aristotle influenced them, but I am not sure in what works.
The concept of substance found its way firmly into the work of all the Continental Rationalists. They may have modified it and understood it differently but it is definitely present in all of them : and all work from the central notion of substance (ousia) deriving from Aristotle : Metaphysics, Γ ,2, 1003a33-b19; Metaphysics Z ; 1, 1028b2-7. (Basically books  & .)
▻ MIND OR SOUL (1)
Psuche is the Aristotelian term; and however critical Descartes was of Aristotle, he very plainly adapted Aristotle's 'ship' image : Aristotle considers the possibility that the mind is related to the body as the sailor is to a boat (de Anima, II 413a8-9). Descartes remarks pointedly that 'I am not merely present in my body as a sailor is present in a ship, but ... I am very closely joined and, as it were, intermingled with it' (Meditation 6, Cottingham tr.).
▻ DISTRUST OF THE SENSES
In the allegories of the Sun, the Line and the Cave in Plato's Republic, VI-VII, the message is plain that genuine knowledge requires a turning away from the world of the senses, infected as it is with change and decay, towards the unchangeable, transcendent, intelligble world of Forms (eide, ideai). This turning away from the senses is just what Descartes does when he relies on 'clear and distinct ideas', invulnerable to error, as the sole foundation of knowledge.
▻ MIND OR SOUL (2)
Descartes insists in Meditation 2 that is the same mind that senses, imagines thinks and wills : the mind is essentially 'simple' or an undivided unity. This is precisely Plato's view in Phaedo 79c-80b. (Plato's doctrine of the tripartition of the soul (Republic (435e-441c) and Phaedrus (246a-b, 253c-254) appears to cut across this but we should note the point that in the final book of the Republic Plato expresses doubt whether the soul is really diverse in the way that tripartition presupposes (REp. X. 611b).
▻ INNATE IDEAS
In Meditation 3 Descartes uses the notion of innate ideas, thus reviving a notion mooted in Plato, Meno 81a-d, 85.
I have concentrated on the influence of Plato and Aristotle on Descartes. There's isn't room, I'm afraid, to extend across to the other Rationalists though there is the point made at the start that the Aristotelian notion of substance influenced all the Rationalists in one way or another.