In a/the:

• another country
• backseat of someone else's car
• canoe
• downpour
• elevator
• middle of a crowd
• movie theatre
• photobooth
• shower
• snow

On a/the:

• birthday
• beach
• boat
• couch
• New Year's
• parent's bed
• plane
• rollercoaster
• train
• wedding day

Under a/the:

• bridge
• fireworks
• mistletoe
• stars
• water

At a/the:

• concert
• front door
• museum
• park
• restaurant
• stoplight
• top of ferris wheel

While:

• against a building
• arguing
• cooking a meal
• in front of a bonfire
• laying in the grass
• posing for a picture
• saying goodbye
• saying hello
• slow dancing
• upside down