I've been thinking about what to do when my brother comes to visit with his family to my house. It's always a struggle between going somewhere "special" (i.e. touristy), or just doing normal everyday activities that are still enjoyable.
No matter what you decide to do, the most important part is spending time together and enjoying that time while you can. Unless there is a specific attraction that your brother & sister-in-law want to see that's nearby, I think a "low key" week together is going to be as amazing as anything else you might choose to do!
Enjoy!!
It will be low key and on a budget :D W will do some 'Finnish' things and go to a few local places that are nice to visit at slow pace. This city isn't really a tourist destination but it is a good representation on Finland.