Most international visitors use Johor Bahru as a transit point only, and the city does not have much to offer other than a number of shopping malls with lower prices than nearby Singapore and an not-so-old, defunct palace somewhat to the west of the city centre.
Johor Bahru is also notorious for robbery and theft, so watch for your belongings, especially near the Larkin bus terminal.
Having said that, with too much time on your hands, you can spend a day exploring the sites of Johor Bahru. Singaporeans frequently visit Johor Bahru for their weekend shopping, so either leave early or expect long queues at the border.
Johor Bahru is also the southern gateway to the Malaysian Peninsular. From here, you can connect to virtually any point in the entire country and to the beach islands in the state of Johor, among them Pulau Sibu, Pulau Besar and Pulau Rawa.