He was a sugar factory manager during Dutch colonialism in Central Java, Indonesia, about mid-19th century, so he got this huge colonial house in the middle of rice field. He seemed to be an influential man; I was told everyone in the village knew him (he was local, btw).
We used to go to this house once a year, but it's going to be sold to real estate developer (which means everything's gonna be demolished completely).
Now, since this is a quite old house, I suppose there are a number of historical tidbits which can be preserved. I'm going over to the house this weekend.
If I were a historian, what should I do/what should I gather to document this house? Besides photos and videos of course.
You need to think not only about documentation and preservation, but also about access to researchers and/or the general public. Museums and libraries are your best options, but they don't accept everything. Large universities often maintain archives and collections as well. These institutions all have limited resources, however, so they often need to turn down artifacts that don't fit in with their current collections or which aren't adequately documented.
I'd recommend sending out a few emails. (I prefer email to contact forms, since it lets me keep a record of what I've already written.) I've listed a few sites below and linked to their contact pages. Note that the most helpful navigation links are usually at the bottom of museum websites (links at the top are more often geared for visitors). I'd also recommend checking in with the libraries and museums closest to your ancestor's house, and perhaps thinking about whatever institutions (including universities) are close to you now. There might be local archives or collections on Dutch colonialism or 19th-century architecture that would love photos or anything else you could provide. Send them the same details as in your OP, and ask if they'd be interested in anything from the house and what documentation they would need.
National Archives of the Netherlands – Note that they're most interested in official government administration, so they might need more details on who your ancestor was.
Rijksmuseum – Check out the very last FAQ.
National Museum (Indonesia) – This site didn't work well for me, although perhaps there are better pages available in Indonesian.
And if there's any particularly interesting bits of the house, there might be specialty collections that would love to get them. For example, there's a number of museums with permanent door knob collections.
That might get you started. I hope you find someone interested in helping you preserve some artifacts from this house. The more details you can gather (with references) about the people that lived there, the better. Thanks for thinking about the future of the past!
I think a solid suggestion would be to get into contact with a local museum. They would probably be a good first step.