Bike buddies - for teens
When my kids were little, dh and I had bike buddies that attached to the back of our bikes and turned them into tandems for one adult and one child. Now that our kids are older, I've been looking for something similar but sized for teens or adults for when I have to pick up the kids and they don't have a bike with them. On occasion I've let my daughter sit on my bike rack but she's not so light anymore and I'd prefer a solution that has her pedaling as well. I'm not going to buy a tandem for the occasional lift, but if anyone has seen any relatively inexpensive attachments, I'd be interested in hearing about them.
Annie

Xtracycle
Check out the Xtracycle:
http://www.xtracycle.com/
http://www.xtracycle.ca/
Good option, but the passenger doesn't pedal. Or Xtracycle + e-assist.....
Trail a Bike
Try these:
http://www.trail-a-bike.com/product/trail-a-bikes/
My son gets a thrill out of pedaling both of us on this - ours is 12 years old.
None that I've heard of!
From the research I did, these trail-a-bikes all have a max weight of about 75 pounds.
It would be hard to make one support the weight of an adult. Trailers can carry a lot of weight, but trail-a-bikes usually attach to the seatpost, probably to facilitate turning. Unfortunately this means that they can't carry as much weight as a traditional trailer. Carrying the weight of an adult would likely require attaching to the frame somehow. The attachment would still need to be near where the seat tube meets the seat stays. It would be more complicated.
It may not even be possible to build one that would work with the wide range of frames on the market. If one was for sale, but didn't work with aluminum frames (for example) it would cause problems.
Hopefully someone will take this challenge and make such a product, because I think that there would be a market for tit.
A tandem is probably the best solution. You can look up some tandem conversion projects on instructables.
It seems like a sensible idea
It seems like a sensible idea to develop a detachable tandem for adults ... so I wonder why it has never been done? Is it impossible to come up with a structure that would support weight and balance correctly, without putting undue strain on the main bike?
Tandems
I don't want to buy a whole other bike just for the occasional child pick-up, but if I did, then this is the one I would want:
http://www.bilenky.com/viewpnt.html
With a tandem like that, I would definitely be the more popular parent for kid pick-ups. :)
Annie