I have been trying to figure out a similar problem. One thing that does annoy me with these options is when you DO want to clip in you can only do so on one side. I hate fumbling when I realize i have it on the wrong side to clip in. That said, a good solution is/would be very useful.

On one bike (a mountain bike) I have a regular old Shimano XT pedal, but since I sometimes want to ride in regular shoes into town, etc, I got a pair of platform adaptors. They work well enough, but are not great.
http://www.amazon.com/Shimano-SPD-pl...+pedal+adapter

I am planning to try try Shimanno XT Trail peddles (or maybe the XTR to save a few ozs and better quality) - this is for a bike where I will be clipped in MOST of the time
http://www.amazon.com/Shimano-XT-PD-.../dp/B0058ZCDHU
http://www.amazon.com/Shimano-XTR-PD...f=pd_sim_sg_44

I have also thought of trying some other peddles, such as the Crankbrothers Mallet series - really a downhill peddle but have a big platform that look like could be used unclipped.