I think they are both must sees.
The actual Anne Frank House is enclosed inside the museum. You get to walk through it as part of the visit. I first saw it in 1977 when it was just the house. I was given a typed-up information sheet to read as I walked through. There was only one very well-used English copy. I was the only person there. That was very moving, but I learned much more this past summer when I visited it again. If you go, look closely at the walls near doorways. I don't remember exactly where it was, but there is a place where the heights of the kids are recorded. That really got to me. Most people didn't even notice. I didn't the first time either.
Keukenhof is amazing in a totally different way.
Here's a link to the official Keukenhof website. If you explore it, you will see how things work. You walk through all the outdoor displays and you can also visit many greenhouses to see the flower varieties. If you want to get out into the bulb fields, you can take a Whisper boat. I haven't done that. Next time.
It's not all tulips. In fact, depending on the weather there may be few tulips except in the greenhouses. If you can be there for the Flower Parade (Bloemencorso), be sure watch it. And take a deep breath when the floats with hyacinths go by. They smell so good!
Most online sources promote viewing locations that you have to pay for, but you don't have to do that. I've seen it twice. In both cases I was just standing by the side of the very narrow road it goes down in locations with very few people. I had a car both times and oddly, parked on a side road near the Roodemolenweg / Sportpark in Sassenheim both times. This is the route for this year's parade. You don't need a car. If you plan ahead, you can take the bus.
Have fun in the Netherlands.