What to do with unused open drain pins
These are really two different question:

How to handle unused open drain pins, and
How to properly terminate I2C bus lines

For the first part it's easy: you simply do not connect them. Or you can also tie the to ground if it helps (usually for thermal conduction). The internal structure is simply a MOSFET with the source to ground and the drain… open (and some ESD protection structure, usually). Just don't tie it to something else different than a ground.
As for the second question, there is a somewhat detailed explanation in the I2C specs (download from NXP) but in short there's and easy rule: pull them up somewhere to the relevant supply with some kiloohm of resistance.
I2C is not impedance controlled so it's not really important where or how you do it. It could be argued that more or less on the middle could be slightly better. The bus itself is multimaster (or at least it can be) so there's not a preferred signal generation point. So just place the pullups where convenient.
As for the value, that is somewhat important. The pullup resistor create an RC circuit with the parasitic track capacity. In short, a bigger resistor causes a longer rise time which could violate the timing specs. A too small resistor would overload the bus drivers.
Unless you have a really long or populated bus or you are doing high speed I2C it really doesn't matter. Popular values are 2k2 or 4k7 ohms. In case of trouble check with a scope and tune them.