You've gotten some great ideas, some I might think about when my daughter and son are older!
I like the photography idea, but how about portraits of just you and her, to capture this moment in her life as she launches into the next phase (sounds like the REAL world has arrived!). You all could get makeovers beforehand or get pendants identical to each other and meaningful.
Will she be setting up a household of her own? Can you get a little welcome party arranged for her and solicit small but useful gifts from everyone in her life, to help set her up?
It'll be so hard for me, I know, but I think one of the most important things for a parent to do at this stage is to let them make their own decisions and mistakes. Writing her a letter about this part of your relationship would probably be very meaningful for her! Good luck with this part!
It sounds like she's been missing the little sisters, because she is thinking about putting their names on her own body, so maybe she'd like a weekend saturated by good times with you and them (indoor water park, Chuck E. Cheese's, shopping for them, visiting family together).
I don't know what's out there, but there could be online or real-life support communities for people leaving the service. If you spend a little time looking up such things, it might give her something to check out and make this transition. Sometimes it helps to know other people who have been through the same things, and just chat and exchange stories.