Suggestions for bike trainer workouts

I've just invested in a magnetic trainer for my road bike. I'm hoping it will be an asset to my training, as it's often hard to fit in rides because of schedules, weather, etc. Now I can spin away while catching up on my TIVO -- truly the best of both worlds

Any suggestions for workouts in the 1/2 to 1 hour range? Equipment wise, I've got a basic heart-rate monitor as well, but my bike computer is linked off the front wheel, not the back.

Typically I've done stationary rides something like:

5 minute warmup
4 minutes at 70-80% HR, 1 minute easy (4-6 total sets)
5 minute cool down

Goal wise, I'm concentrating on building base for the spring, with a plan to do 3-4 sprint distance tris and 1 Oly this season.