Suggest a rainy day trainer workout?

I'm new to road biking and training to try the most mileage I've ever done: a 50 mile ride on June 1 with a decent amount of climbing (2500 ft, but much of it is in one or two significant climbs). I'm up to about 30 miles of training and 2500 feet, but my rides keep getting rained out. So it looks like an evening on the trainer for me. I'll have my heart rate monitor. Any suggestions for what kind of ride to do or what HR zone to stick to? I haven't ridden in two days and want to keep my legs fresh.

Television w/"commercial break" intervals!

I'll watch an hour long show and whenever a commercial break comes on, I'll do some sort of interval- could be high cadence, high gear (low cadence), to a certain heart rate, etc.