Here are some ideas.

1. Throw a bake sale
2. Have a family yard sale
3. Have a donated or new book sale
Suggestion: Use an heirloom keepsake book or well made scrapbook. (see profile)
4. Have a kiddie carnival
5. Hold an auction

Combine them all.

Some folks don't write checks or handle cash daily anymore so setup your paypal asap or get a website that takes payments like the ones reviewed here... http://family-reunion-planner.fimark.net/familyreunionwebsite.html

Send online payment ready emails to each family member to make the payment process easier.

Offer rewards for the first 10 0r 20 payees. Special seating at the reception. certificates, a baked item or our favorite, an heirloom like the keepsake book https://www.createspace.com/3601499 The book contains a family reunion planner, family tree, pedigree, family recipes section, journal and reunion themes and games.