I find hills to be a good "natural" intervals training, too.

However, what is a good workout depends on you. I rode 50 km with a friend who's quite untrained yesterday at a very slow pace yesterday and when I told her I was going to do some real-training today, she said "What? I thought THIS was quite a training!! What do you call a training session?" The answer is: an hour an a half with warmup and a few periods of intervals, with some racing against myself on the Grand Prix course (perfect pavement).