syntax_error:
What do you guys think about one of these?
Serial Interface Board Module
These (given some trouble with the various alternative pin allocations, a hot topic on the "Displays" forum) are a popular part of the Arduino repertoire. Trick is - you need a four pin connector and cannot make the cable too long. For longer cables, you would want to use customised I2C functions to run the interface slower, but perfectly do-able.
syntax_error:
I also like the db25 cable. Are there any other cables like that with less pins? I suppose USB can work too (for temp and float sensors).
USB is tedious to use with the Arduino, other than via the USB-TTL adapter which converts it to serial so you would be just as well to use plain serial (TTL levels). What you can do consistent with your original request, is to use the USB fittings for something non-USB such as I2C, as long as they are not accidentally plugged into the wrong thing.
syntax_error:
Edit: Never mind. I just saw that there's DE-9, DA-15, DC-37, and DD-50 cables.
Wow! I am impressed - you know the proper names for the "D" connectors, despite the errors in those descriptions.