I think lying about your activities can get you into some actual trouble - so this could be risky advice if they follow up on who you were visting.
What is extremely worrying is the comment that 'they have a checkbox for that'.
What the hell are they doing recording the reasons for movement of law abiding citizens?
They don't know that you're law-abiding, for one thing. Law enforcement agencies routinely record information which is volunteered to them.
Guilty until proven innocent, eh?
Also, his comments in the follow up about refusing to answer to avoid the possibility of that answer being used later against you (no matter what you say) do have legal merit, but I don't feel that they are entirely practical. I think it more likely that they will attempt to concoct charges against you for refusing to answer their questions (drawing both attention to yourself and their ire) than answering the question in the most innocuous way possible.
I hate saying it more each time I do, but the best practical defense against law enforcement run amok is always to draw as little attention to yourself as possible.
I believe there's a Martin Niemöller quote that'd be quite apropos here...