Keukenhof and Delft in one day?

Yeah it's doable. Keukenhof is open until 7:30pm (last entrance at 6pm). So you can spend the morning and early afternoon in Delft, then take the train to Haarlem (it's on the same train line) and from there the direct bus to the gardens. Best to visit Delft on a Sunday when the outdoor markets are operating. Also best to purchase combination tickets which include the garden and bus tickets ahead of time via this link:

http://tinyurl.com/of8w2sp

If I did that I would do it around the other way. Keukenhof can be quite tiring - there are miles of paths and flowers to see everywhere so there can be quite a bit of slow walking and standing. Some people take only 2 or 3 hours there and others take longer.

You can get there directly from Amsterdam.

If you get there when they open you have a fair chance to see some before the tour buses arrive. It is often crowded, even more on the weekends and around holidays or on school half term, so if you can plan your arrival day all the better.

Then when you have had enough tulips and hyacinths, windmill, food, photos, and bulb purchases, you can take the bus to Leiden from whence it is a very short train ride to Delft. Or, if you take the bus to Haarlem you can see the growers' bulb fields as the train passes them before it gets to Leiden, and then you have the short ride to Delft where you can do as much as your remaining energy allows.

To visualise the train routes around there, Amsterdam Centraal is at the top of the diagram with Haarlem just to the west. The line from Amsterdam goes south through Schiphol to Leiden and onwards towards Den Haag, Delft and Rotterdam.

The line from Haarlem (which also connects to Amsterdam Centraal among others) parallels the main line a few km over towards the dunes and has a few rural(ish) stops before it joins the main line just above Leiden, and trains from Haarlem continue until at least Den Haag. The growers' fields which are visible from the train are on both sides of the line from about halfway from Haarlem until just before Leiden.

You can get there directly from Amsterdam.

There are no direct buses from Amsterdam to Keukenhof. One has to take the train from Central to the airport and then bus to the gardens.
By visiting the gardens (or any other popular tourist attraction) late in the day one avoids the tour buses and crowds on any day of the week.
If you go to Delft on a Sunday afternoon the market stalls will be long gone.

Thanks for the suggestions. Would it make sense to rent a car for the day?

You're welcome. Stick with the trains and buses. If you rent a car you would probably wind up parking at the Delft train station anyways:)

Renting a car would give you the freedom to drive around in all kinds of places. We stayed in Leiden and drove to the Flower Parade one day, as well as around the fields. That day and weekend is probably the busiest of the whole season for Keukenhof, so we went to the gardens on the following Monday. For an idea about the driving, you can go to my blog entry documenting it: http://travelswithtrout.blogspot.com/2013/07/benelux-april-2013-driving-to-and-from.html. You are welcome to poke around any of the other posts if you are interested. Click on the pictures to make them bigger and to be able to just run through them without the text.

You don't say when you are going, but this was the 2nd time I've been in the area for the Bloemencorso and the 2nd time the weather leading up to it was so cold so late that there were very few tulips to be seen on the ground or on the floats. There were tons of them in the greenhouses. It was also quite cold for the parade and when we were at the gardens.

We didn't go to Delft, largely because I had been there before.

"If you rent a car you would probably wind up parking at the Delft train station anyways:)"

There's actually quite a bit of readily available parking in Delft along the ring road. But for the most part, you can't take a car into the historic core of the city.

If you consider this daytrip, I would get an early start and try to arrive at Keukenhof as early as possible after opening. By mid-day, some of the indoor displays can be packed. By the time I left, even some of the outdoor garden areas resembled a human version of a German Stau.

And this all pre-supposes that your visit coincides with the relatively narrow time frame that Keukenhof opens each year.

I will admit that we drove to Keukenhof, but that's because we spent two weeks driving from Cologne to Amsterdam, via Venlo, the year there was a Floriade there. Anyway, the tiny local streets were jammed with tour busses and independent-spirits in cars. It was like arriving at Disneyland during a school holiday. And we had to park on muddy grass in the overflow parking. I'd rather have been in a bus. Exiting was also time consuming, with special left-turn rules, and so on.

Although I understand your desire to be efficient, I don't see these two stops as being connected. For myself, I'd give Keukenhof most of a day, but I didn't, it would combine with Haarlem, the Aalsmeer Flower Auction, or maybe Leiden. Delft goes with Den Haag, Rotterdam, or Kinderdijk. That said, I spent a whole day in Delft as a train run-out from Amsterdam, but my objective was to be home for dinner in Amsterdam, not to hit two places in one day.

Appreciate the tips on Keukenhof and parking. We are going first week in April getting car for a day, stopping to enjoy the flowers enroute to Antwerp final stop Brussels then leaving car. Any other tips much appreciated, first time to Netherlands and Belgium.

@norskdame I would seriously rethink your plans if you want to do this all in one day. You will spend most of the time in your car, not at the sights you want to see, and will probably arrive in Brussels at midnight. Keukenhof is unique--give it most of one day and stay nearby in Leiden or Haarlem. The next day visit Antwerp on your way to Brussels (although I would stay in Antwerp, since I like it better than Brussels, but that's just me). Also, are you aware of the extra fees for renting a car in one country and dropping it off in another? Train travel might be better, especially on the trip from the Netherlands to Antwerp and Brussels.