I've been at BON a few years ago, but only as a stopover on an SXM-AMS flight. We had to disembark for refueling, but I never left the terminal building.
Back then, we landed from the east, on runway 28. There is a perimeter road, if you have a car you should be able to just stop there and take photos from the road side.
Same for the western side of the airport, there's a road there as well. The terminal itself is pretty useless for spotting.
Keep in mind that there is very little traffic at BON, no more than 10-12 movements per day, and that already includes the local puddle jumper services to CUR. So make sure you only go out there when the KL A333 or the Thomson Dreamliner are due.
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