週末、映画を___と思っています。
Q. Which form expresses 'I'm thinking of watching a movie'?
Translation
I'm thinking of watching a movie this weekend.
Explanation
The volitional form + と思っている expresses a current intention: 見よう (volitional of 見る) + と思っています. 見ると思います would mean 'I think (someone) will watch' — a prediction, not an intention.