Recommended: The Mercure Hotel - we have no special rates but it's definitely the closest hotel to the venue and it has the best breakfast in Amersfoort - with freshly made omelets and some of the best rooms. 10 minute walk from the station and has a parking for 10,- EUR per night.
Also close by: The NH hotel - also has parking, 10 minute walk to the venue and 5 minute walk from the station. Not as luxurious as the Mercure. https://www.nh-hotels.com/en/hotel/nh-amersfoort
The city center has a few hotels but be warned - the city center also has lots of pubs and they only close around 4-5 AM. At 9 AM on Saturday, the market starts and there are churches with really big bells nearby. Much cheaper than the Mercure but also less luxurious.
If you decide to get a hotel a little further away (Campanile, Bastion, Van Der Valk) then you can park underneath the venue (Parking Eemplein) or you can park in the residential area just east of the venue, across the water (don't worry, there's a bridge so there's no swimming involved). Don't worry - all these hotels are just 10 minutes away from the venue.
Amersfoort also has a limited amount of AirBnB possibilities. Also close to the venue is the ONLY camper-parking of Amersfoort. There are only 3 spots and at least 2 are usually taken. If you arrive a little early (on Wednesday or Thursday), you may be able to snatch on of them.
There is also Vita nova: a Bed and Breakfast "on the water" just a few minutes walk from the venue - it's very small and it's almost always booked so if you want this - be quick!