Split is primarily for walking. The main site to see is the Diocletian's Palace. Here's a snippet of what I wrote about it from a trip report:

It’s an amazing place built around 300 AD and containing some 220 buildings that are now homes and contemporary shops, such as Benetton and Mango. Diocletian had the place built as his retirement home. Apparently, he only ventured out to watch Christians get thrown to the lions. The place is wonderfully preserved. There’s a peristyle courtyard, an Egyptian sphinx and a belltower you can pay a couple of dollars to climb up. Ironically, Diocletian’s mausoleum was eventually turned into a Christian church and his body was removed. The octagonal building now has an interesting relic-filled treasury. There are pieces of skulls inside metal head-shaped containers with viewing windows on the top, boxes of bones, pieces of fingers, and purportedly a couple of pieces from Jesus’ cross. The basement is also open for a couple dollar admission fee. It used to be the garbage dump – those above would put their trash down holes to the basement, but it’s been cleaned out and you can walk through and see the wonderful vaulted ceilings. It also houses contemporary art work.