Well I love the north of Bali - Tulamben. It's out of the tourist bustle. There is not much to do but scuba and chilling. You can hire fishing boats quite cheaply for transportation to other dive sites. It's fun to rent a car and randomly explore the coastline to the NW of Tulamben. Certainly there are no technical diving facilities or He available, but... plenty of very accessible shore diving on the SS Liberty wreck (to about 30 meters) and there is a very deep "Drop Off" wall.
A nicely kept secret are the $22ish bungalows with hot water, aircon, ocean front and set in a coconut forest at Puri Madha (or $7 for a cold shower room) - It's an older place just NW of the public access beach (go to the access and then walk West along the beach to find it). It’s a more difficult to find from the main road as the sign is small and old – you can try to head to the far NW side of town, just past the access beach, then turn down a long dirt road. It’s well out of the tourist hustle with$5-$6 tanks available on demand any time of the day. If this place doesn’t appeal to you, then just show up in town and look for a place that suits your needs - there are plenty. Several are upscale, but not as low key. But be aware that there are no ATM's in Tulamben and probably no internet or it’s very limited.
Padangbai, where Absolute Scuba and many other shops are based, has many more amenities and a small night life. The access dive sites from S to N of Balli. The town is also a favorite of mine for chilling ;-). Plenty of places to stay and eat, etc.
I’d split my time between both sites and maybe another as well. A week in Tulamben might be enough.
Be sure to save a few topside days to explore the culture, food, shopping and Wyan’s massages at “Bodyworks” Spa in Ubud ;-).
Have fun!