Keukenhof , Maastricht

"Best time of the day to go?" As soon as it opens. My wife and I arrived in the morning, and for the first 1-2 hours, we were able to move about freely. By lunch time, the place was packed. You could barely move through the indoor displays.

I was there the last day of Keufenhof a couple of years ago, and while there were a lot of people, it was all very manageable; didn't have any problems with overcrowding in the buildings. Was there from about 11am till 3pm. To get there from Amsterdam take the shuttle trains from Centraal Station to Schipol Airport, from Schipol there are special buses non-stop to the gardens. The buses are found directly outside the main entrance of the terminal. There will be reps from Keukenhof in the area to point you in the right direction.
You can purchase combo bus/entrance tickets at the official site: http://www.keukenhof.nl/en/477/entrance.html

I wouldn't try to do it as a day trip from Maastricht. While there is an hourly train from Maastricht to Haarlem via Amsterdam it isn't the fastest train in the world, taking about 3 hours to Leiden. I've never been at the last week - we are usually there around the time of the parade - so I can't speak for the crowds, but I would have thought that they would be less than when the gardens are at their peak. I've never found "good value", if what you mean is cheap (its ok to be blunt so we don't have to try to read your mind) in Maastricht. We like to stay nearby in Aachen, Germany.

Last day is usually more packed, but if it is the only day available, go for it - as soon as it opens. No need for a tour whatsoever, buy a combo ticket that includes shuttle buses from Schiphol bus station, go to Schiphol by train.