I'm struggling with organising a picnic for some friends. Can you help?

What a thoughtful friend you are! You might pick a theme that might tie in with the park, thus justifying your park pick. If no theme, pick a place that everyone will go to. I like the idea of keeping the activity simple. The treasure hunt is a great idea and can be adapted for all ages. After you do the hunt and eat, everyone will want to visit. Or you could have the hunt start before you eat and have some things hidden with the meal. Have fun! It sounds like a lovely plan!