Your friend invited you to a picnic which you enjoyed very much.white him a postcard where you: express your gratitude for the invitation, praise his mother's food, praise their hospitality, express your pleasure of having great fun.

Hello Joe !
How is you doing ? I'm rellay glad that you are invited me. That was really cool picnic this sunday!  I've enjoyed it very much! Your mother's food was gerat and your hospitality was perfect! I also enjoyed having fun when we are decided to play some footbal after this. Dont you want to repeat this in the next holidays ? we can meet near my house and have some fun too! 
Write back soon.
Kevin