Go to a nature park and walk the 5 mile (3k) trail.

Camp out in your backyard.
(With friends, and alcohol)

Sit on your rooftop at night.

Design your own constellation with someone special out of the stars in the sky.

Wear a wig in public all day.

Find a special spot somewhere within walking distance of your house. Then after everyone is asleep sneak out of the house and go there to escape everything and think about life.
(At home mine was under a pine tree to the side of the house. In Korea it was at the helipad on top of a hill away from everything. In Georgia it was under a lightpost by a big open field.)

Handcuff yourself to your girlfriend at school and don't bring the key. It works better with real handcuffs because they can't be broken open.

After payday, you and two or three of your friends take off work and go on a roadtrip. Don't discuss where your going until your pulling out of the driveway.

Play in the rain.

Find a very tall place like a watchtower in the park and stay there until sunrise with some friends or someone special.

Talk in an accent.

I've done all these and they're great.