Grammar fix
What are you up to this weekend?
ask weekend plans
A friendly, casual way to ask what someone is planning to do at the weekend.
expressionB1core
Hear it in context
Notice where the phrase sounds natural.
So, what are you up to this weekend — anything fun?
A: What are you up to this weekend?
B: Not much — maybe a walk if it's sunny.
Fix the pattern
One small grammar move makes it sound right.
Not: What are you up to for this weekend plan?
Say: What are you up to this weekend?
'Be up to' already means 'doing', so don't add 'plan'; just say 'this weekend'.