The BEST way to get a summer pen pal is to have a friend or family member set you up with one. If you have a friend who is spending the summer elsewhere then ask him or her to be your correspondent for the season.
There are also places online where you can "meet" a pen pal (Google "How do I find a PenPal to write to?" but only do this with PARENTAL PERMISSION).
The trick to having a pen pal is that you write actual LETTERS and MAIL them! You can even set-up a local pen pal with secret mailboxes that you deliver your handwritten letters to. You may even want to set up a secret mailbox system for your family and let everyone write letters and draw pictures for other family members all summer long.
Community Engagement Adventure
For the community engagement level Adventure set-up a neighborhood summer mailing system with other kids in the neighborhood.