How about Square One... Mathnet! The Mystery of the Maltese Pigeon was my favorite one. I remember it so fondly, I'm almost afraid to go find it on YouTube.
As a kid, I remember watching one episode where they opened a door with a credit card(!) and I was like "Mom, do you have a credit card" - I didn't tell her why and next thing I know after thirty minutes I shocked her by opening our side door with the credit card. Later, I got it down to 5 seconds and they changed the locks...