The enquiry is a bit too general, but I'll try to give some input on Bali. Consider staying in Kuta (crowded tourist area) or Nusa Dua and Seminyak (if you prefer a quieter area).

Join a local tour so you can see places around Bali, such as Bali dances, silver handicraft producers, Tanah Lot, Lake Sentani, Ubud (be careful of your belongings as monkeys will try to steal them, I'm serious), Goa Gajah, Tampak Siring, Pura Besakih, Kintamani, and Trunyan Village (This may be disturbing to some as dead people in this village are not buried. They are exposed but do not generate bad smell caused by decomposition). May not be able to visit all within 4 days so you need to prioritise because Bali is relatively large as compared to Phuket for instance.

Consider hiring a bicycle to travel around too .