California is epic, my favourite place I've ever been to, though NY was also great. Vegas I found was fun for a couple of days, but underneath the glitzy exterior I found it was quite depressing really, would go back for a lads weekend but can't say I feel drawn to go back.I cant decide between NY or California/Vegas. Both very different but i'd love to visit both. Not sure i'd be brave enough to go alone though, not great at meeting new people.
If you went to Cali on your own, I'd make a trip of it, go from San Diego up to San Francisco and stay in beach hostels, you'd have an amazing time. Equally, spend a while in NY and there are great places to go and see beyond the tourist traps.