Luxor has a few indoor spots including a brick wall that gets exposed to a fairly strong spotlight from the distance; good spot for creative experiments. Most of the other areas inside the Luxor require a really fast prime and/or bounced flash.
Many couples like to stop by the Vegas sign, just south of Mandalay Bay. Venetian has a red wall by the front entrance and good architecture, but the canal shoppes inside may require that you stop shooting unless you get permission beforehand. Wynn has various spots inside and out, TI has an outdoor spot, Bellagio and Paris both have nice outdoor spots including fountains, the water show, etc. Bellagio has the conservatory, not enough light in there at nighttime and not much to bounce flash off of, but they have a sunroof/skylight which gives very adequate light during the day. Big tall escalator/tower on the south edge of the bellagio for some good shots. Lots of lights on the opposite side, some bright enough to give adequate light for nighttime shooting without flash.