could and could have

to make suggestions we can use on pourrait + an infinitive verb.

On pourrait jouer au tennis. - We could play tennis.
On pourrait aller à la plage. - We could go to the beach.
On pourrait visiter un musée. - We could go to a museum.
On pourrait faire un pique-nique. - We could have a picnic.

To express what we could have done, we can use on aurait pu + the infinitive. 
(Pu is the past participle of the verb pouvoir - "to be able to.")

On aurait pu jouer au tennis. - We could have played tennis. 
On aurait pu aller à la plage. - We could have gone to the beach. 
On aurait pu faire un pique-nique. - We could have gone for a picnic.