The cheapest and easiest way is to travel by train.
Departures from Ljubljana Train station at: 2:35, 6:15, 8:35, 13:20, 14:08, 15:40*, 17:45, 21:05, 21:45.
*not on Saturdays and Sundays
The journey takes aprox. 2 hours. Price (one way): 11,90 eur
Ljubljana train station phone: +386 1 2913 332
Web: www.slo-zeleznice.si/en/international_transport/international_links/croatia/zagreb/#