Hey Eva,
Certainly Gili islands are a good choice for diving but Bali has excellent dive sites both for beginners as well as advance divers. Why spend extra money on going to Gili just for a few days ? ( my personal opinion, boring haha )
Anyway, on the island of Bali you have a few options which Neptune operates as well:
1.
Padang Bai - the south-east corner of Bali. There are a total of 9 dive sites just around the cost with a a boat ride of maximum 5 to 10 minutes. As you will be having your OW certification, certainly you will have to go to Shark Point just outside the port of Padang Bai where you can see the White Tip Reef Sharks.
Padang Bai is famous for variety, underwater macrophotography... small stuff but again, you have to try Shark Point.
Underwater conditions, low or almost no current, excellent for beginners as there is where most of the dive centers are doing their PADI Courses.
2.
Nusa Penida - this trip practically you are leaving the island of Bali heading to Nusa Penida. Probably the highlight of scuba diving in Bali, you will see the majestic Manta Rays at Manta Point and beautiful coral and underwater fauna at Crystal Bay.
Despite all articles regarding Nusa Penida, underwater conditions are good for even Discover Scuba Diving there so don't listen to everything that people say

)
3.
Tulamben - recently open after the volcano situation, this is where you can see the famous USS Liberty shipwreck as well as the artificial dive site called Coral Garden. Hot dive sites with water temperature year-round at approximately 29 degrees, this is a shore dive.
One part of the wreck is located at approximately 15 meters deep so your OW certification will suit you perfectly for this dive site.
There's however a pretty long ride from south of Bali to reach Tulamben, estimated 2 hours but absolutely worth it as you are going up the mountain and then down the mountain to reach the east coast of the island. The scenery is majestic
Overall, these are the best worth mentioning in an initial conversation but there are quite a few more dive sites: Amed, the rest of the dive sites at Padang Bai and perhaps the drift dives at Nusa Penida.
With respect and appreciation,
Julian