The G-Shock GW-M5600 is a pretty good starting point. Classic looks and it's relatively slim with the newer case design
I can't think of a square 5600 styled G with a thermometer. Most of the thermometer Gs tend to be pretty chunky to be honest. About the 'smallest' I can think of is the older G-Shock DW-6100 which isn't really that small.
If you do need thermometer functionality I'd go look at the G-Shock GW-9200 Riseman
The Riseman also has a barmoter which I find quite useful and which more importantly works in wear. The thermometer function of any of these watches doesn't actually work on the wrist due to body heat, in order to take an accurate reading casio recommend you take it off for 30 mins.