Well good for you! This is a wonderful gift under any circumstances, but particularly in view of the ones you describe.

I did something similar for my wife for her birthday several years ago, when I took her to Paris for her first visit there. No guarantee this would work for you and your other half, but for what it's worth here's what we did:

1. She kept her earbuds in with some music playing the whole time, and kept looking down as much as possible.
2. If I recall correctly, at check-in I approached the desk agent without her and explained the situation. The agent agreed to go along with it. I believe my wife had to join me at check-in, but the agent didn't tell her where we were going and gave me the boarding passes. (Though fro this stage I wonder: Can you simply check in and get your boarding passes online?)
3. I can't recall how we handled security, but generally they don't say where you're going when they check your passports and boarding passes anyway.
4. Boarding the plane, my wife kept her earbuds in and music playing and kept her eyes down while we boarded. It wasn't until we had our glasses of bubbly in our hands that I told her.

Granted, your situation is more complicated by virtue of the connection, whereas we were flying non-stop. But I hope this info helps.

Have a great trip!