You could try....

I would recommend Cosmo Hotel.  Nice rooms and great view of the Bellagio waterfall.  They have real nice balconies and you will see lots of other ladies on TER use the terraces for their photo shoots.  But then you said you don't want ultra modern and if there is anything that Cosmo is it would be ultra modern.  You may want to try Paris or New York, New York. Decent rooms and not to modern.  Encore is also nice but modern rooms

I had some great photos taken at the Venetian a while back. The colors and prints in the rooms is warm and romantic.  

flamingo is old school Vegas , with modernized real cool rooms, and Bally's next door is retro 60s remake.  
near strip Tuscany is cool, but small, very small
Bellagio is definitely not ultra modern, old world appeal and has a beautiful gardens exhibit
ceasar's palace is well, the grand daddy and has a nice pool area in back as well as nice shopping mall
the rio is getting ghetto, but still has appeal if the gangsters aren't hanging
downtown the D is kind of retro, and is refurbished, and you can't beat the FOUR QUEENS
if you want something old vegas, try old Binions down town (old style gambling hall type saloon)
NYNY has a nice re make of coney island and is not ultra modern
if you want the replica's of Europe try Paris or  Venetian, if you like to walk like an Egyptian try LUXOR they have cool , almost endless wading pools (or they used too)
if you want MOSCOW try The RED SQUARE at MANDALAY BAY
if you like pirates....rrrrrrr....try TI (treasure's island)
I know nothing about the re make of old Nazi palace, I mean IP (imperial palace).
Aria , city center and the COSMO are all ultra modern

if you want to go off strip. the ORLEANS is pretty cool,  
so is the PALMS (really nice pool) but it's small

The M CASINO out in Henderson and the Ranch are different, modern and not. but far from the strip

the Strat is a dump in my opinion, Tropicanan is old school and was recently re modeled haven't been there
the Rivereria and Circus - Circus aren't what they used to be.
The frontier is long gone (but I hear the workers are still on STRIKE)
the WYNN- PALLAZZO - and the the new casino next to the WYNN( the ENCORE is all red inside, Zen like)
There is run down, oh yeah, the HARD ROCK off strip, celebrates the history of Rock and Roll.
Hope that helps narrow down your selections

You Las Vegans are awesome!

Thank you all for your help. I have some great leads and I'm excited to book my shoot! Thanks for all the help both here and via PM.