Why do these things happen III

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I have agreed to go with my W and S to her parents house next weekend, she invited me, and thru IL's found out it was her idea.
We will be having a BBQ next sunday, which is Fathers Day, and happens to also be W's birthday. With everything I have read and learned, I am not suppose to do anything about birthdays and such, so not to look clingy,pushy, etc... I am not sure I completly agree with this tactic, but since I am new I think I should follow most of the advice.

Here is the question, how do I follow these DB'ing rules when I am invited to go out of town, 4 hour drive. It is Father's day and her birthday. If I get her something, it could be wrong and I realize this, but what if she does something for me about Fathers day? Do I just stick with Bday card? Get something not overly expensive but that she would like and use? Do I get pnumonia and just back out of trip?