Maybe he should get that on video instead.
Prepare her a nice dinner followed by a movie or a relaxing oil massage. Throw in some candles and flowers and your set. Leave the parents at home. You can also make her a bday card. Another idea is to make her a personal coupon book.
With discount sheets for condoms, oils, lubricants, jellies, handcuffs, and candle wax. 
Why not just ASK her what she would like to do? When, where, how? Tell her your budget and note the time constraints so if she wants to sqeeze in something tomorrow fine - lunch maybe - but if she would rather wait until the weekend for the dinner and park walk or whatever . . . fine. She IS your girlfriend and should understand the time and other constraints including the budget thing. If she doesn’t understand and isn’t willing to help you out by giving clear hints such as coming right out and telling you so you don’t have to guess or go online to get awkward and potentially harmful advice from complete strangers on an internet board then . . . well . . .
I’ve never had a girlfriend who wasn’t understanding about such things . . . but then I tended to cut angstgirls out of the loop before such opportunities arose.
she’s your girlfriend . . . just look her in the eyes, tenderly hold her hands and as whisper sweetly and oh so romantically . . . “sweetheart, where would you like to have your birthday threesome? you, me, and another hot babe” . . . er, sorry, I believe that may have been my birthday fantasy. Might not work with her her . . . then again . . .
Since you’ve stirred the pot . . . do let us know what happens.
All the best,
Brian
[quote=“meiguolangren”]Why not just ASK her what she would like to do? When, where, how? Tell her your budget and note the time constraints
…
I’ve never had a girlfriend who wasn’t understanding about such things . . . [/quote]
I have…man, the stories I could tell ya…