Las Vegas: Book for mid-May through mid-August. According to Hotwire, those rates can fall as low as $55/night. The heat may deter many tourists, but the air-conditioned indoor activities will make up for the sweltering dessert conditions. Find Las Vegas hotel deals here.
Orlando: Book for late August to see rates drop to around $42/night. Year-round, rates hover around $50/night, but generally peak around the holidays (more specifically New Year's Day) at $76/night. Find Orlando hotel deals here.
Los Angeles; San Diego; Washington, D.C.: Book for January to March and October to December. These popular cities display extremely similar hotel pricing trends and have their highest rates in spring and summer.
No matter where you're going or when you're booking for, always shop around for the best deal. Also, use tools like Bing's Flight Search which will tell you whether you should book your travel or wait as fares may lower a few days later.