While most locals will be happy to see you at a temple, you should be aware that their religious ceremonies aren’t meant as tourist attractions.

At the temple, please dress modestly. To enter you need a sarong that covers your legs and a shawl or top that covers your shoulders.
The following are mandatory for all to enter a Balinese temple:
Sarong (also known locally as kain kamben) around your legs.
Temple scarf (known as a selendang) around your waist.
These items are usually rented out at most temple entrances but feel free to bring your own. If you are invited to a ceremony, you should wear the religious clothing that the locals wear, and you should seek advice from your host as to what you should buy.

Respect sacred territory, don’t enter or trespass. This includes holy spots, and for women, it’s not appropriate to enter these sites when menstruating.
Taking nude or indecent photos at sacred places is regarded as defilation.

Balinese locals are far more conservative than most Westerners; they frown on public displays of affection. So when in or near Balinese temples or rural settlements, please keep PDA to a minimum. The same goes for clothing: dress as modestly as possible, especially when visiting temples.

Finally, photography is fine at religious sites in Bali, but flash photography inside sacred places is less appropriate.
If in any doubt, ask somebody what’s the best way forward, and keep smiling.