I have two cheap magnetic trainers. They are not wimpy. You change resistance by using the gears on your bike, and the speed of pedaling, and by the increase in magnetic field available through a cable that attaches to the bars.
It really helps to have a trraining DVD or two. Actually, they give one quite a workout, if you do as Coach Troy says.
That said, I HATE the trainers, and will do just about anything NOT to use them.
I have fitted out my old mtn bike with headlights and rear blinkie, and have figured out how to keep warm, pretty much. So, whenever possible, I will take an early morning ride.
Also, I sort of enjoy the spinning classes at the rec center - be prepared for lots of "older" ladies in great shape who will show you up. They do a LOT of standing, which I don't do while riding, and am not used to. Entirely different muscle set.
However, in a spinning class one can "cheat" just a bit, pretending to turn up the tension knob while not really doing that!!
You can also do exactly what you want and sort of ignore the instructor.