First off....

ON THE NEW PUP!!!! 2nd....PICS PICS PICS!!!!!
Now my o2 on your question. Your doing the right thing as far as keeping it fun. A young pup has to have fun 100%!! In the past when I have this problem (and I wouldn't realy consider it a problem at 12 weeks) I always train with a check chord. Make the dog come all the way back to you, and not stop short. Much praise when she gets back, and defenitely let her hold for a bit. Right now, I would keep play time just that....PLAY TIME!!! The only training I would be doing is OB OB OB. I game of fetch every so often is great, but I wouldn't be running any sereous retrieving sessions. Focus on OB, and keep the sessions 15 to 20 mins (depending on her attention span), and KEEP IT FUN!!!
Not sure what your experience is with training, but if you plan on doing this yourself, I would rcomend investing in "FOWL DAWGS" with Rick Stawski. This was recomended to me a couple years ago by some experienced trainers on tis forum, and has helped greatly. There is a specific order of training that a pup needs to follow. Make sure you do not move on to the next step before the dog is solid on the current one. Training is not a race. Take it at a pace that the dog dictates, or you will have to hit the rewind button and move backwards.
KEEP SESSIONS SHORT (2-3 15 minute sessions a day are much better than 1-1hour session) KEEP IT FUN (as soon as you or the dog start getting frustrated stop immediatly) TAKE IT SLOW (it's not a race, just because Joe Blow had a dog that had it's 1st retrieve at 6 months doesn't mean you will. You'll end up with a much more solid dog if you spend the required time on each phase, and the dog dictates how long that will be)
GOOD LUCK!!!! KEEP US POSTED!!!!! PICS PICS PICS!!!