Add the following sed commands to cal to get a calendar with the current date marked:

cal | sed "s/^/ /;s/$/ /;s/ $(date +%e) / $(date +%e | sed 's/./#/g') /"

If your version of grep supports coloring matches, you can use the following to give a calendar with the current date colored:

cal | grep --color=auto -E  \(\ \|^\)$(date +%e)\(\ \|\$\)\|\$