When something is "on tap", it means it's convenient. Hot water on tap, cold beer on tap, etc. You can extend this to mean you're in a situation where a certain thing is convenient or easy to acquire. Money on tap, love on tap, entertainment on tap... For your phrase, I'm thinking of bars which might put a different beer on tap every week as a special offer. So as Angela and Jennifer explained, "what's on tap for today?" is basically asking what is planned for today.

"On tap" : planned (as an idiom ) So, it means "what are you going to do today?" ... "what is on your list of things to do for today?" ... "what is your schedule for today?" ^^

"what's on tap" 1. Definition: What’s going to happen? or What are your plans? Examples: What’s on tap for tomorrow? Are you going skiing?