I have the compass sensor as well which is fun to play with. Since we're a big soccer family, I'd like to get the infrared sensor and IR ball since we have two NXTs, we can have family Robocup Soccer matches!

I have the old LEGO Ed thermometer for the NXT but heard that LEGO ed has made a new one which hooks directly to NXT and will be more metal to withstand chemicals and higher/lowertemps - gosh darn it.