Trip in mid May to Netherlands and Belgium

I think you can easily do justice to Amsterdam and surroundings in 5-6 days, depending on how much of the surroundings you want to see. If you stretched it to 7-8, you'd probably still find new things to do every day and be able to take more day trips. But you might also want to consider relocating to Delft or Utrecht for a couple of nights in that case.

Keukenhof dates for 2018 show that the last day is May 13, so if you arrive before that, you can still go see the tulips there, though it will be the end of the season, so not at their peak.

For Belgium, again it depends on how much you want to see. Assume the other three major cities (Antwerp, Ghent, and Bruges) get a day each, plus a day for Brussels, then consider how many other day trips you want to take.

We visited for the first time in April of this year.

We spent 5 nights in Amsterdam (it was our arrival city, giving us 4 1/2 days) and daytripped to Haarlem, Den Haag and visited Keukenhof. The weather was cooler when we were there, so we didn't really venture out in the evenings - and I feel like the day trips took away from seeing Ams itself...and there were so many other day trips we could have done! I think you should look at where you'd like to daytrip to and plan accordingly. If I was staying and only exploring the city itself 2 or 3 full days would suffice, but toss in daytrips and I think a few more nights are needed. ( I really want to return and spend a few days just in the city itself - hopefully when the weather is a bit warmer). In Ams itself, we did a canal tour and visited the Rijksmuseum (same day) and that was it other than wandering.

After Ams, we spent 3 nights in Dordrecht, daytripping to Gouda and Kinderdijk.

After Dor, we spent 3 nights in Ghent, BE. We stopped in Antwerp for an afternoon and loved it. From Ghent, easy daytrips to Bruges and Ypres. We only had a few hrs in Brussels in before catching our train to Paris. Ghent was nice and centrally located. We really enjoyed Belgium.

Can't speak of ease of getting into Ams - it's pretty easy from the airport. We stayed at an airbnb that was about 5 min tram ride to Rijks and a 10 min walk to the Albert Cuypmarkt, so we were kinda central.

5 or 6 days enough time to see most of Amsterdam and the surrounding area?

Yes.

How many days would be sufficient to see Brussels and the surrounding area?

You should base your self in Antwerp or Ghent 4 nights and do day trips to Brugges and Brussels

the Element is not in the city in Amsterdam

It is in the city but not the old center. You will see that Amsterdam is bike oriented. It would take you only 15 minutes to ride a bike (away from all the other car and bike traffic in the center) to the Museumplein where the Van Gogh, Rijksmuseum, etc are located.
Or you can just take the tram number 5 a short walk to the Element.

Thank you for the responses. We hoped to visit Haarlem, Leiden, Gouda, Utrecht, The Hague and Delft, Zaanse Schans and Giethoorn if possible while in Netherlands. From what I have heard you can possibly do 2 or 3 in a day. Would staying in The Hague be a good base?
In Belgium we had thoughts of visiting Ghent, Bruges, Mechelen, Leuven, Tournai, Antwerp and possibly Mons.

We only did one location per day. But hubby is not an early starter, so most we didn't get to until 11am or so, staying for the afternoon then heading back to Amsterdam.

I had thought about Zaanse Schans, but a lot of people say it's quite touristy, so we opted for kinderdijk, which was great (but very cool when we were there).

In Den Haag (The Hague) we went to Madurodam (so much fun...even though we're grown adults) and the MC Escher museum.

In Gouda, unfortunately we went on Easter Sunday, so 99% of stuff was closed, but we picked up a walking tour guide at the cheese house and followed that...it was quite informative and fun and took a few hours. We were also able to go into the town hall (for a few euro) and looked around.

You have a lot of places on your list. It might be possible to group several into one day trip, but if I may share my experience in the Netherlands...

I planned a day of hopping from one town to another via the bus (http://www.localbus.nl/). I started by going to Hoorn, and then I went to Edam. I spend quite a bit of time walking around those two towns, and by the time I was finished in Edam, I was ready to return to Amsterdam.

Your experience might be different, but I think it's hard to visit too many different places in one day, as you will either run out of steam or run out of time (or both).

And with all those Dutch day trips, don't forget that there's a lot to see in Amsterdam itself. With 5-6 days, you'll probably want at least half of those to be doing things in Amsterdam. So that gives you at most 2-3 days for day trips. I think Haarlem is easily worth a full day, as are The Hague and Delft (which you could do together in one trip), and Keukenhof is a day trip which you could potentially combine with Leiden. That doesn't give you any time to get to the other places on your list.

In Belgium, I think Ghent, Bruges, and Antwerp are each worth a full day, as is Brussels.

If you can take more time to see all the places you want to visit, by all means do! But don't try to cram too much in to too little time, or you will regret all the running around you did.

I should add that if you want to do some bike riding (for instance, you could ride bikes from Amsterdam to Haarlem or to some of the towns north; and you could rent a bike in Bruges and ride along the canal to Damme or beyond), that's going to take even more time.

Dutch commuter trains make it possible to do two in a day, but that is hasty, check-the-box tourism. I find Brussels less attractive to sleep in, but the museum quality is very high. The average streetscape, not so much. Belgium also has easy, cheap commuter trains. Lines to buy tickets can be annoying.

Thank you for all your input. We have decided to spend 6 nights in Amsterdam and will see if we can get about 3 day trips in. Had a friend tell me Delft was a must. Then we will move on to Belgium and spend 5 nights in Ghent and visit a few cities around there.

Sounds like a great plan! We enjoyed visiting Gravensteen castle in Ghent.