I would like to go from Munich to Stuttgart on a weekday by coach/train - day Return. How much is that approx?

Try this website, http://www.db.de/site/bahn/en/start.html.

You can enter in your information about the dates and times. I entered in generic dates and times and for the ICE train it will cost around 95 euro for 1 adult, 2nd class.(round trip) For regular trains it would cost about 75 euro for 1 adult, 2nd class (round trip). I would suggest if possible to go during the weekend and buy a weekend pass for Bavaria for only 20 euro. You can go any where in Bavaria all weekend for this amount and it is good for up to 5 people.

First of all you can't get discount cards for one day, they're only valid for a year and not worth it for returning same day.

I had a look on www.bahn.de for you, Just putting in the 28th june as an example. the train leaves at 826am and arrives in stuttgart at 1047am, then leaves at 1958 and arrives back in munich at 2220. the cheapest ticket is €63.75

Unfortunately I don't know of any bus companies who do regular trips to Stuttgart and I've only found charter ones so far.

PS If and only if you were to change your mind and go at the weekend I would recommend a Schönes Wochenende ticket which allows you to travel from midnight saturday till 3am monday and is for unlimited travel, it costs €30 so saves a lot

but if it's a weekday then fair enough

hope you enjoy stuttgart