J. - there are daycares that will allow drop in care, which basically means that you pay the registration fee, and then when you need care, you call them and see if they have room. This could be risky if you're calling the day of to see if there is room, but if you know your schedule a week or so in advance, it really shouldn't be any probelm to call up and tell them what days you need care.
There is usually both an hourly and a day rate, which would cap at about 3 or 4 hours - meaning they charge, say, $12 an hour for up to 4 hours, then it's just a flat $48 for the day or something.
I don't know what area of town you're looking for care, but if you PM me & let me know, I'll search our database for you and tell you which ones do drop-in care.