Bike Rental in Haarlem

Just return it the next day. Second, ask the rental for security suggestions. All of the bikes we rented came with a frame lock that locked the rear wheel but that doesn't prevent someone from carrying the bike away. So, from home we bring two of our heavy duty cable locks to lock it to something or as a min to lock the bikes together. Harder to carry two bikes than one. We have never had any problem but most of the rentals are clunkers so not sure what the theft potential is.

We've generally found the quality of the rental bikes adequate for our purposes, but then we're not avid cyclists at home and thus aren't looking for anything fancy.
All of the rental shops will provide a lock when you pick up the bike, and will be happy to show you how to best use it to secure your bike while you're off doing other things. Our impression was that theft wasn't a big problem, but best to inquire locally when you collect the bike in order to be aware of potential problem areas to avoid.
The bike shops will all have a cycling map highlighting some of the best routes for a day's exploring. It'll cost a couple of Euros but we thought it was well worth it - found several "bicycle only" routes that kept us away from motor vehicle traffic and which generally made for a more pleasant experience cycling thru the surrounding countryside.
And don't forget some padded cycling shorts, especially if you're only casual cyclists. Trust me on this one.

We rent bikes whenever we're in the NL. Blackbike or Macbike in Amsterdam - not sure if they have locations in Haarlem. Lock comes with the bike and Ive never used an extra lock. Usually put the lock through the wheels is enough. The bike shop will give you more feedback about safety. Biking is really the best way to get around the NL - you'll have a great time