Funny story: this episode being about Marshall's bachelor party turned out it was the weakest storyline. Sure it had some funny moments but I think they could have done better with the idea, pushed it further, like they usually does.
I really found the Robin / Lily / adult - playing - thing - with - battery storyline better! The inappropriate gift idea is a classic but it was well played. The grand-mother's speech was priceless and the conversation with the eyes between Robin and Lily (and it's outcome) really made me laugh.
And how about what Barney did for Lily? I'm not saying more aside from what was already said: He is the bestman. Ok, co-bestman. Blink.