Secret Places (8)
Climb the box, jump onto the slanted side of the building, and then jump onto the ledge under the "Innocent?" sign.
Jump onto the slanted side of the building, and then jump into the second or third window.
Within the same building, open the "Attack of the Bleached Blonde Biker Bimbos" poster.
In the theater lobby, activate the cash register. The wall that opens is much too high to reach. Press on the wall under it to activate a secret elevator (as seen in the image).
This secret can be accessed from either the bathroom vent or the projector room (press on the wall by the fire extinguisher).
Jump on top of the projector to open a secret wall.
Blow up the center of the movie screen to access the secret place behind it.
This one is quite hidden. From the bridge at the end of the level, you can jump to a ledge which leads back around to the front of the movie theater building. Follow the ledge until you see a tree. You can use the top of the tree to jump at the window and find this secret place. If you have a jetpack, you can simply fly into the window.
Unofficial Secret Places
Jump on the rubber bin beside the arcade elevator and press against the corner. The bin gives you the extra height boost needed to reach the secret.
Pressing on the "Duke Nukem II" arcade game incites Duke to spout an infamous one-liner, and a hidden compartment containing a holoduke to reveal itself. I included this one for the sake of completeness, and because there may actually be some poor uninformed sod out there who really didn't know about this.
Use a pipebomb in the bathroom vent to blow open a hidden crevice.
Use a jetpack to fly to the top of the back alley. Watch out for the two turrets up here. I can't believe this isn't counted as a secret place.
"stryker metronet.com": On the baseboard of the toilet stall with the defecating Assault Trooper. This was Allen Blum's e-mail address at the time (minus the "@" symbol, which does not exist in the character set).
"867-5309": On the shoulder of the left washroom urinal. This is in reference to a popular song from 1982 called "867-5309/Jenny" by Tommy Tutone.