Accommodations in Zagreb

Do you have a price range? If not, the Sheraton was nice - a short walk from the main square (this was from 8 yrs ago)

I stayed at Hotel Jadran a while back and it was quite reasonably priced. My room was in need of remodeling, but was fine for one night. Their website suggests the hotel may have been remodeled recently, and prices are in line with your range. Positives are that it's in a quiet neighborhood about a 10 minute walk from the center, room price includes a nice buffet breakfast and free off-street parking is offered. You might also consider Hotel Dubrovnik. It's more expensive, but has a great location on one of the pedestrian-only streets in the center.

We stayed at the Best Western Premier Hotel. It is a short walk to downtown or the train station. It also included breakfast.

I don't know if you have stayed in hostels before, but when I was in Zagreb I stayed at Funk Hostel. Very clean and nice and very cheap. My 3 friends and I stayed for $35 each p/night. Shared toilets and showers though. I know some people get kinda freaked out by that prospect, but it was really clean and nice. The staff is super friendly and helpful, too. Here's the link:
http://www.funkhostel.hr/