Reading sensor data over MUX
The DS18B20 is a 1-Wire device and doesn't require a multiplexer, each device has its own address on the bus and you can connect a lot of them with a single wire. Depending on the humidity sensor you pick you may be able to use the same bus for it as well.

It is harder to find 1-Wire humidity sensors, but you could use a DS2438 Smart Battery Monitor which also includes an ADC to give you values from an analog humidity sensor.

The challenge of designing your project will be setting up an easy to understand display and controlling how/when it changes.