How would a waiter say "What would you like?"

I believe that "You would like X" == Ba mhaith leat X, but I'm not sure how to turn that into a question.

On the flip side, I believe "I would like a pint of Guiness, please" == Ba mhaith liom pionta Guiness, le do thoil.

Any corrections or suggestions appreciated.

In the standard, ar is the question form of ba. So, 'Would you like X' becomes Ar mhaith leat X

Cad é/cad/céard ba mhaith leat?

or
Cad/céard a bheidh agat (le n-ithe / don bhricfeasta)? = What will you have (to eat / for breakfast)?

And "What would you like to drink?" be Cad ba mhaith leat le n-ól" or "Cad a bheidh agat le n-ól??

There's a grammatical difference
Cad ba mhaith leat a ól/ a ithe = What would you like to drink/eat? (without le but with a)
Cad a bheidh agat le n-ól / le n-ithe? = What will you have to drink/eat? (with le)

le is part of the phrase with the verb bí: bheith le ... ag ... (having to ...).
So if you would like to use le with ba mhaith leat you must also add bheith:

Cad ba mhaith leat a bheith agat le n-ól / a bheith le n-ól agat? = What would you like to have to drink?

But you don't need to use bheith le with ba mhaith leat
"cad ... a ól" is a sufficiant verbal noun phrase (ba mhaith leat just interposed).
Compare with a possible answer: Ba mhaith liom beoir a ól = I would like to drink beer.

Thanks all for the answers

Yes, that is correct. To turn it into a question, you can say An bhfuil sé/sí? which means "What would you like?". For example, An bhfuil sé/sí leat? translates to "What would you like?".

I'm not sure why you think "an bhfuil sé leat?" means "what would you like?" It doesn't mean that at all. It actually means "is it with you?"