If you're looking to improve your bike further, have the mechanic give it a quick inspection while you've got it at the shop, to see if it would benefit from a tune-up. If the Pacific is in its native state of assembly from Wal-Mart or wherever it originated from, this could yield big benefits in longevity, safety and functionality, if the mechanic does a conscientious job.
Hopefully you have a helmet, otherwise that should top your list (get it at a shop and have them do a complete fit adjustment)
Other than that, if you don't already have a waterbottle and holder, that would be a great thing to pick up (if the frame has a position for it, otherwise consider a Camelbak water-pack). If you ride in poor light or at night, a Cateye headlight and blinkie would be smart. If you don't have a good lock and you need one, pick up a lower-end Kryptonite U-lock maybe.
As for parts for the bike... as you know, Pacific isn't an enthusiast-quality bike, so don't get carried away pouring Corvette money into a Pinto. Get your Pinto tuned up, maintain it, yeah... but save up the Corvette money towards a serious bike with oil-damped forks and a frame that fits you just right