"go downtown", "tour the southland" and "go under the hood", are slang phrases meaning to perform cunnilingus i.e., to perform oral stimulation of the clitoris or vulva.
Assuming it is performed in all corners of the world, we would expect a list of synonymous slang words and phrases to be very long and impossible to produce. From The Urban Dictionary and The Online Slang Dictionary I can cite the following:
go down on
dine at the Y
tip the velvet
eat a furburger
eat a peach
eat fur pie
eat some clams
have a box lunch
munch on
square away
muff-diving
Obviously these are all vulgar and whoever uses them should know exactly when and where it's acceptable, in addition to who they are talking to.