Well, I always wanted to take my band into a Toronto subway station
during the time when the subway is out of service (0230-0500), since
the stations have a very nice reverb!
Failing that, I always wondered what the results might be if I put us
at a certain location under an overpass I knew...it had a very odd
echo in one place where sounds actually repeated themselves in short
pulses three or four times per second...
Finally, there were a number of different studios and record firms
that used their (tiled) rest rooms as "echo chambers," with speakers
to feed the sound in and mikes to pick it up including echo! I don't
know what happened if someone used the facility mid-session, which
would have added interesting adjuncts to the recording...?!