A trainer is a very good investment - just a simple model, not the fancy computer controlled ones from Tacx. Put a movie or some music on and you are good to go for 1h-1.5h. Currently I am still riding outside and I do weight training 2x a week. When it starts snowing I am dusting off the trainer to substitute for my hours on the bike.

Spinning is a nice addition since most trainer workouts are steady state. A spinning class has a bit of everything squeezed in - tempo, resistance, etc., and is nice to see some people around every once and a while. =)

Good luck!