hmm, as i said before its rly up to you,any of places has all that you need..
since im from Zadar ill give you some hints how it is here..
beaches, also depends what you like, there are rocky beaches, sand, etc... also, outside of city u have small places like Zaton,Nin,Petrcane, Punta skala etc ( just google em, for example, zaton is a big tourist camp with lots of caffes,restaurants, shop, very big sand beach with like zillion people, most tourists that come like to go there)
fun, i think u wont be bored here nor anywhere, when its summer season u can find fun on every corner.. also there is some paintball just outside of city, like 5 min with car..but other than that, everywhere u go there is lots of ppl,parties,etc etc..
history, as of Zadar, 3000 year old city, i think u wont be missing any history
i mean i was with my bro and his gf one day trying to show them and explain all, and still couldn't do it in 1 day... not to forget other coastal cities/places, u rly wont be missing any history
(zadar,split,sibenik,vodice,makarska,trogir etc etc, or some places in Istria lika Pula,Rovinj,Porec and so on)..your best choice is to google those places, or any other u like..
http://www.zadar.hr/index.asphttp://www.sibenik-tourism.hr/eng/http://www.vodice.hr/en/index.phphttp://www.pulainfo.hr/en/http://www.visitsplit.com/15663/en/i gave you URL's of some places in english version ofc..