Here are some other ideas for making the ride harder -- these will also freak people out:

1. Adjust your brakes so they rub
2. Remove a spoke or two so the wheel is way out of true and rubs the brake
3. Ride with flat tires
4. Ride with knobby tires
5. Ride with studded tires
6. Put your seat really really low
7. Just take the seat off and stand the whole ride
8. Put on some ape-hanger bars off an old Stingray
9. Fixed-gear conversion (okay, this might not freak people out)
10. Get a bike trailer (only $70 at Target) and tow it. Put a cooler of cold drinks in it, and you'll be the toast of the group!