The stay's for the 11-13th April (checkout on 13th, so nights of 11th and 12th). That's a Monday - Wednesday.
If you were to book before March 11 you could use the 30% off advance booking promo at the Tropicana and grab a paradise suite for $350 taxes in. An executive two room suite at regular rates is only $414. All of the rooms are renovated and its within walking distance of other properties. Take a look at this while you're doing your research - I haven't stayed here but have heard that the new rooms are pretty great.
We would like to stay at the Cosmopolitan.
I checked smartervegas.com but they don't have any promos listed for Cosmo. The best they have are up to 20% off at Aria using "ARMIRJ1" and a $50 daily resort credit using "AR7D30" (buy before March 13) - I'm not sure if these are better than what you currently have..?