We recently had Autumn's 1st birthday in a park and I'm really glad we did. Other than gathering everyone for the candle blowing/song, I didn't organise anything else (eg. games for older kids etc.) and didn't need to as everyone was so happy just playing. We had people bring a chair or blanket though. Kids food is easy, adult-wise I asked a few people to bring a hot water thermos and just provided tea/coffee/sugar/milk and this worked well too. If you keep it super simple, you'll have more time to enjoy it yourself. The writing down of presents on the cards is an excellent idea if you plan to open them at the park. And the best thing is going home to a clean, quiet house for nap time!