Formal:
* Que diriez-vous de... (What would you say to...) - This is the most formal option and is often used in written or professional settings.
* Que pensez-vous de... (What do you think of...) - This is also a formal option, but slightly less formal than "Que diriez-vous de...".
Informal:
* Et si on... (What if we...) - This is a very common way to suggest something in informal situations.
* Pourquoi pas... (Why not...) - This is another informal option that implies that the speaker is open to the suggestion.
* On pourrait... (We could...) - This is a less forceful way to suggest something than "Et si on...", and is more appropriate for making suggestions that are not too demanding.
Here are some examples:
* Que diriez-vous de manger au restaurant ce soir? (How about we eat at a restaurant tonight?)
* Que pensez-vous d'aller au cinéma ce week-end? (How about we go to the cinema this weekend?)
* Et si on allait à la plage demain? (How about we go to the beach tomorrow?)
* Pourquoi pas faire un pique-nique? (How about we have a picnic?)
* On pourrait aller faire du vélo après le déjeuner. (How about we go for a bike ride after lunch?)
The best way to choose the right phrase is to consider the context and the level of formality you want to convey.