What a great opportunity to come to The Netherlands - I live in The Hague so my comments will have some bias towards the NL. Amsterdam is a wonderful city to visit and also a great jumping-off point to see many other parts of the North and South Holland districts. Thus, five days is great and enables you to also visit Utrecht, Haarlem, Leiden, Rotterdam, and other locations easily by train.
I've visited Giethoorn a few times - primarily for weekend stays. We drive from The Hague as Giethoorn is in the middle of nowhere! (We also take our bikes. Like most parts of The Netherlands, it's a great place to ride - you can rent bikes & boats in Giethoorn). We found numerous parking lots available. On our first trip, we stayed at Hotel Brasserie De Pergola (pretty basic), another time at a B&B, and also caravanned with a large group and stayed in a house. All of the accommodations we sourced were 2 or 3-star properties. We did rent a boat on one trip - had a blast. If you can avoid staying there on a weekend, you'll have a far better experience as it tends to get crowded. We generally spend two nights - which really equates to 1.5 days on-site. Not sure I'd see this as a day-trip as we found a lot to explore in the area via bike. But I could easily understand this as an "arrive early in the day/leave late the next day" type of location and thus be a one-night stop. (not a lot there if you remain in town).
The Cochem region sounds like a great choice... we been a few times (mostly on the way to somewhere else) and we always drove, stopping to explore when we found a town we liked. I suppose the number of days you stay here would be impacted by how many locations to visit or how much exploration you want to do.
I'd agree that Berlin and Munich deserve a separate trip!
As far as returning to drop off a car, you can certainly opt-out for Frankfurt, but you'll pay drop-off fees. Perhaps trackback through The Netherlands, stopping by Maastricht for a night, then ending up in The Hague. If you're interested in US military cemeteries, you'll find the Netherlands American Cemetery six miles from Maastricht. And I love Delft and ride my bike there at least once a week and visit the markets on Saturdays - but it's just not big enough for a two-night stay.
Rather, spend your last few days in The Hague. Here you have the Binnenhof, Escher in Het Paleis, Panorama Mesdag, Madurodam, Mauritshuis Museum, Louwman Museum, The Peace Palace, the Kunstmuseum Den Haag, The Hague Forest, and a compact "Old Town" city center. And if all else fails, head to the Scheveningen Beach - a short ride from downtown via multiple trams.
From The Hague, you can drive to Kinderdijk or train to Rotterdam... and you can ride the #1 tram (or train) from downtown and be in Delft in about 20 minutes. I would suggest trying to plan to go to Delft on a Saturday morning. Each Saturday, there's an Antiekmarkt Delft (antique & flea market) alongside a farmer's market. The stalls are spread along the canals and it's a great way to spend a morning and find unique gifts!
May is also a great time to go to FloraHolland - it's just after the height of the season and they'll be busy. For a change of pace, head to the Kröller-Müller Museum, where you can get inches from the 2nd largest private Van Gogh collection in the world - and they let you take photos! Also, it's a great national park to ride bikes around.
I suppose I'd stay with your circle route... but lean more towards the NL (Again, I'm admittedly biased. You could easily spend two weeks here!). Perhaps an Amsterdam, Cochem, Maastricht, The Hague type of trip. From The Hague, the ride - by car - to the AMS airport is all 4+ lane highway and takes about 35-40 minutes. By train, it's only 29 minutes to the airport.
So much to see - so little time. Enjoy your trip!!!