If you want your riding to be harder, wear looser fitting clothing. A normal or baggy T-Shirt, baggy shorts over your riding shorts (so that you still have the padding)... put a front pouch on your bike to store your stuff.
The less streamlined you are, the harder you'll have to work. If you typically do a 25 mile ride and average 15mph, then de-streamline and do that same 25 miles at 15mph, you've really just done the streamlined work of 30-35 miles in the same amount of time as you used to do 25.
The key to exercising is making the exercises gradually harder.
My workout regimine starts August 21, after I get back from taking my girlfriend up to college. I won't see her for about 3 months and want to try and WOW her.
Cycling: 3-days/wk, starting: 90mi/wk, goal: 150 mi/wk
Weights: 3-days/wk, starting: bench 75lbs, goal: 125lbs
Ab-stuff: 2-days/wk, starting: 200 crunches/week, goal: 600 crunches/wk
(yes I know that adds up to 8 days, some days I'll be doubling up)