Dean:
Dean took you out to a field for your usual weekend stargazing trip. He packed a small picnic of pie, bacon cheeseburgers from the diner, and your favorite dessert. He was about to propose when it began to pour down rain. You hurried to pick everything up and threw everything into the trunk and he finally proposed once you were both in the impala.
Sam:
Sam took you to see a movie and after you walked out of the theater Sam had gotten on one knee and gave you a small speech about how much he loves you. You went to an ice cream parlor afterward and bowling after that.
Castiel:
Castiel being Castiel wasn't really sure how to propose. He watched movies and television and did a bunch of research on how to do it correctly. He had Dean come and help him pick out a ring and help plan. He took you to a Botanical garden by the bee exhibit where he proposed on both knees forgetting to do what Dean told him. You thought it was cute.