Step 1: Recall the rule of indirect speech.
When the reporting verb ("told") is in the past tense, the verb inside the reported clause must generally be shifted one tense back.
Step 2: Apply tense change.
Direct speech: "I \underline{am thinking}…" → present continuous.
In indirect speech (past reporting verb), present continuous becomes past continuous: "was thinking."
Step 3: Adjust time expression.
"This weekend" changes to "that weekend."
Step 4: Construct final sentence.
Rafi told Mary that he \underline{was thinking} of watching a film that weekend.
Final Answer:
\[
\boxed{\text{was thinking}}
\]