Just by reading the question title, my straight, undoubted answer was consúltalo con la almohada. After that, I saw that was your proposed answer, and I think it is the best option, as it respects the sense of taking a rest and freeing your mind before taking any decision.
The exact meaning is as follows:
aconsejarse, o consultar, con la almohada
- locs. verbs. coloqs. Meditar sobre algún asunto, aplazando la decisión durante un breve tiempo, normalmente por un día.
Just think how many times you have find the solution to a problem while your head was on the pillow. Moreover, I can't think of any other option right now. Maybe repósalo or just piénsatelo con calma.
Just out of curiosity, centuries ago there used to be an expression in Spanish quite close to the English one. It's found in the Diccionario de Autoridades (book III, 1732) and in the edition of the DRAE of 1780:
DORMIR SOBRE ELLO. Es tomar tiempo para deliberar mejor en un negócio, mirándolo despácio. Latín. Maturius rem considerare, deliberare.
In the same dictionary but in another book (book I, 1726), it already appears the current expression:
Consultar, ò hacer la cuenta con la almoháda. Es tomar tiempo para meditar algun negócio, y digerirle entre nosotros mismos, à fin de proceder con maduréz en la resolución.
@Flimzysomewhere in your comment. Otherwise, he won't get a notification about it.