Here is a Custom Inspiration Board that I did for someone. They are having a reception for their destination wedding that they had and wanted to incorporate the beach since that is where they were married into a casual picnic theme.
Invitations: Set the theme by sending either a beach or picnic styled invitation; if your guest list is small you could hand deliver invitations rolled up and put in a bottle with sand and a sea shell
Table Settings and Decorations: Have a picnic table set up or have blankets spread out for guests to sit at. For centerpieces, you can use picnic baskets with sea shells and beach items coming out of them. Decorate galvanized buckets with your color scheme and add sand with flowers in it for another great centerpiece. Personalize sea shells to use as place cards.
Food: Serve picnic styled food in boxes for the guests; you can choose either a beach themed cake or beach sand cupcakes for everyone to enjoy.