What is your idea for this?

Make it on a ビーチ :3 And it would be cute if あなた forgot your picnic basket, so あなた two have a romantic McDonalds-Beach picnic. Lol

A simple walk in the park, holding hands and talking.

Romantic ディナー at home/backyard like a picnic (use your imagination) with garden lights and candles and cushions set up as a tropical safari :)

If the guy knows the girl well, he should take her to a place she wanted to go. Like a fun girl. A fun girl would want to go to...
ディズニー World
Water Park
Silver Dollar City (They have good food)
Amusement park
U get what I'm saying?

Well, let them have a picninc in a very romantic place. Like where they can see very beautiful things. (Moon, flowers, river) The environment must be very silent and peaceful cause it wont be romantic if they cant focus on one another.

A ride on the ocean with his ボート if he has one. A picnic on the beach.

How about a walk under the stars, with the guy and the girl wearing barefoot? That's romantic. U can make the scene on a ビーチ または an open field. Then they'll start talking and exchange each others stories. Hope I can help. :)