I agree that sometimes that is a problem, so I find out the latest tmp directory with ls -tr /tmp | tail. But using nicer names would help with that.

So I took your tip and now I have

    cdtmp() {
        local name
        name=/tmp/$(diceware --no-caps -d - -n 2)
        mkdir $name && cd $name
    }

I decided not to worry about the directory existing already, or of the other checks mktemp -d would do for me in my single-user system.. I suppose mktemp -d -t cdtmpXXX would work almost as well. Thanks for the idea!