When I travel north of the Alps, even in midsummer, I always pack a warm layer (at least a fleece jacket; a merino-wool sweater would also work) and a waterproof jacket. It is all too easy to look at the average high temperature for a destination and subconsciously expect that sort of weather every day. Averages hide some pretty miserable extremes. Furthermore, the temperature you experience at 8 AM or 9 AM (or 6 PM or 7 PM) may not closely resemble the day's high temperature. If you get soaked to the skin in the Benelux countries, you could end up being really miserable on a cool day.
I also pack a good-quality, relatively lightweight collapsible umbrella, because I don't want to have to go looking for one if it starts to rain. You can't really trust weather forecasts in places like the Netherlands; rain can show up unexpectedly. I've had the experience of losing an umbrella and having to buy one quickly from a street vendor, as Rick suggests. It as cheap, as one would expect. Unfortunately, it was also junk.
The handle broke off almost immediately and the umbrella completely failed long before the end of the day.
The website timeanddate.com provides actual, historical, day-by-day weather statistics for most cities of any size. The data goes back about ten years. Reviewing the most recent five years' worth of data will give you a decent idea of the range of conditions you might experience. I see that on June 30, 2021 the high temperature in Amsterdam was 61F, and it looks as if it rained for a good bit of the day. Wikipedia tells me that in the average month of June, it rains in Amsterdam on 9.7 days.
TImeanddate.com weather statistics for Amsterdam - June 2021
You can use the pull-down box above the graph to change the month and year. Use the Search box near the top right to find data for other cities.