It really depends on how (and where) you plan to lock the bike. A hotel is different to a campground, a city is different to a village, outside a shop is different to a museum. Also, is it just the bike you're worried about or your gear too?
For opportunistic deterrence I'd advocate a long cable lock (or 2!), preferably with a combination. Depending on circumstances you can lock the bike to something and include wheels and saddle. Depending on your panniers/bags they can be added too.
I also use (non hook) bungee cords that are great for wrapping in spokes and around frames. They're different and hopefully confuse a potential thief.
The best defence against the serious bike thief is not to park it where they operate. In NL that is any public space with lots of bikes - think non secure parking at train stations. They'll have the tools to break your chain lock in seconds. I prefer outside a Police station. Or places with lots of CCTV.
I try to avoid visiting urban museums etc. on the bike unless I can have secure parking.
Playing the hapless, slightly anxious (foreign) tourist will often open secure doors literally. Especially if solo.
I've toured quite a bit in NL and understand the anxiety but I've never had a bad experience - only lots of good ones. Don your sensible head, have a look around and get a feel for the place.
If staying in hotels try to check in advance if they have space for a bike. Older buildings in the bigger cities can have very limited space. And very steep stairs!
Have a great time!
