Formal:
* Rêver un peu: This is the most literal translation and is quite formal.
* Faire un petit rêve: This literally means "make a small dream" and is also fairly formal.
Informal:
* Rêve un peu! This is a more direct and informal way of saying "dream a little!"
* Fais un petit rêve! This is the informal version of "faire un petit rêve."
* Laisse-toi aller un peu: This means "let yourself go a little" and implies dreaming in a more relaxed, carefree way.
To emphasize the "little" aspect:
* Rêve un tout petit peu: This means "dream a very little bit."
* Fais un rêve minuscule: This means "make a tiny dream."
The best choice will depend on the specific situation and the tone you want to convey.