Driving
The regulations vary within the provinces, e.g. rental cars are only allowed in Bali. In other provinces, you therefore have to take a scooter or a driver. If you wanna drive on our own, you should better carry an international driver's license with you.
The development of the road network depends on where you are. For example, the roads on Bali are mostly well developed and in good condition, while there are (still) some very rough roads on Lombok. Some of the roads are damaged by undermining or erosion.
Some of the traffic signs and rules may differ from those in Central Europe. In particular, right-hand traffic applies in Indonesia. The right of way has the person who takes it. Honking is often a sign that you are being overtaken.
The speed limits are 100 km/h on highways, 70 km/h on rural roads and 30-40 km/h in built-up areas. There is no alcohol limit, as people drink very little alcohol anyway for religious reasons. However, an accident under the influence of alcohol can result in a prison sentence. In general, the penalties for traffic violations are relatively high.
Phone
If you use your home SIM card, you will have to expect high roaming charges. These vary from provider to provider. You should therefore get an Indonesian or international SIM card if the Wi-Fi in your accommodation, restaurants or public places is not enough for you. You can buy an Indonesian SIM card on arrival at the airport. Alternatively, I can recommend the international SIM from Airalo (affiliate link).