While I don't believe either of your specific tour-the-airplane requests could be honored - particularly the tarmac walkaround, when the captain needs to be 100% focused on safety - I do imagine a brief cockpit visit before or after the flight or at least a hello from one of the pilots along with a set of wings or commemorative trading card could easily be arranged. Even if she's seen it before, perhaps the pilots could point out unique features of the specific plane they fly...

I am pretty confident the crew would be happy to deliver birthday wishes with a meal service, though. You could consider an LED candle to be supplied with the ice cream sundae, for example, and I am sure they wouldn't mind bringing out a birthday cake you supply discreetly before the flight either.

I'd approach the gate agent before boarding begins and mention the special occasion, and ask if a cockpit tour is possible if the crew isn't too busy. If the GA isnt able to help coordinate, I'd ask the purser once onboard; they're also the right ones to approach about the special birthday treat requests.

Most flight attendants are totally onboard with celebrations as long as they don't disrupt other people's flights, so I'd imagine a reasonable request would be granted. I'd definitely phrase it as "she'd love to meet the pilots and/or see a new part of how the plane works", rather than a specific request they'd be unable to accommodate like the walkaround though... it's possible the crew might have some even cooler ideas if you let them be creative.

(And in the future, if you want an official onboard celebration, book your flight on Korean Air - in J and F, they offer preorderable birthday and anniversary cakes!)