My G is hitting 50,000 miles quite soon, and I was wondering what would be a good maintenance procedure for it. I have had an oil change recently (around 200miles ago), and the power steering fluid was changed out as well.

I had new brakes installed around 3 months ago, so those are going to be changed also. I was thinking maybe the transmission fluid? Any ideas/suggestions? Its a 2003 G35 Sedan 5AT

I would advise looking at the maintenance schedule listed in your owner's manual. Some other items I can think of off the top of my head are inspect/replace air filter, A/C filter, and check battery level. Moderate your tire pressure (on a monthly basis), and perhaps throw in a fuel injector cleaner treatment (preferably Chevron w/ Techron concentrate) to do some good for the fuel injectors and improve your fuel mileage (slightly)

I think the next big maintenance interval is 60K. I'm gonna hit that in 5K more miles .

At 60k you should be replacing tranny and brake fluid. go ahead and do it now if you are looking for a tune up.