The nearest airports are Amsterdam Schiphol and Eindhoven. We recommend Schiphol. Check the cheapest flights on https://flights.google.com
From Schiphol, you can
book a taxi with Taxi Boute at +31-33-258 88 49
up to 3 people for 95 EUR
up to 3 people for 25 EUR
take a direct train that runs every 30 minutes and will drop you at a 10 minute walk from the venue and the Mercure and the NH hotel (see "by train")
The closest train station is Amersfoort Central. There are direct train connections from Schiphol, Berlin and Utrecht (and many other stations)
Check the national train schedule: https://www.ns.nl/en
Check the international train schedule https://www.nsinternational.com/en
You can pay by debit card on train stations and public transport. You no longer need a registered payment method to ride the public transport. Paying by debit card is the cheapest way to travel. Just "tap" to pay at the gates. Warning: the gates at Schiphol are small poles on the airport level. You also need to "tap" to leave the station.
Amersfoort is on the A28/E30 heading North from Utrecht. There is a (expensive) car park underneath the venue and there is free parking at 10 minute walk. There is a cheaper car park underneath the Mercure.
The city centre is car-free/No Emission Zone. Driving through the inner city in any car/motorhome/van results in a fine of 119 EUR. There are underground car parks at the periphery of the city centre.