Where are you staying? Depending on where you are staying, the Las Vegas Monorail would probably be the fastest and cheapest way to the Stratosphere. The Monorail has a stop at Sahara. You would want to get off at the Sahara Station. From there, you can just cross the street over to Stratosphere.
This would work great to getting downtown Fremont Street as well. You would do the same as I mentioned above. After you get off at Sahara, you can take a bus to downtown by way of Las Vegas Blvd. There is stop by Fremont Street. This is where the light show is. Then just do the opposite coming back.
That is awesome thanks! Someone had told me about this bus but said it didn't go up to the Stratosphere or Fremont st. So this is very helpful.