I think that depends on who will be at the picnic and where! You could hand out colorful posters/cards to the people invited or send a message via email/text. If you are inviting your family and/or friends, something colorful and personalized would be great. But, if you invite people/colleagues you don't know very well, something more formal would be the better idea.
When writing the posters, make sure to include information on where the picnic/trip will be, as well as when it should take place, who hosted it and what they need to do to join.
