The key to a test from my book gives the following answers:
It's my mum and dad's golden wedding anniversary next weekend. We've been arranging a surprise party for the last three months. We've booked a room...ordered a cake...received over 50 replies to our invitations. Mum and Dad suspect we've been planning something special, but they don't really know what. I know Mum has been thinking very carefully about their outfits. She's bought a new dress and ....
A couple of students have chosen 'have planned' and 'has thought". Although I'd go for the progressive, I don't think the present perfect is incorrect. Or is it?
Would you prefer one to the other, or does it depend on what you'd like to emphasize?
If you want to emphasize the duration of the planning for the event and for the outfit, the progressive is a good choice.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.