This question means excellent news! If the interviewer is asking for your availability over the next few days, chances are you are moving to the next round. Try to be as flexible as possible with your availability. Finish your answer by asking for the interview!
"I can make myself available at any time, with some notice. The ideal times for me to interview are between 8-9AM and 4-5PM Monday through Friday. I am very eager to move to the next round. Do you have a specific day or time in mind?"
"I would need to take a half day off to come in for an interview, so given a day or so notice; I can make myself available, no problem."
"Understanding things move quickly; I am flexible and adaptable to work around your availability. What day and time do you have in mind?"
"I am so happy that you asked! I have a flexible schedule and often work from my home office. What day and time do you have in mind? I will make it work."
"I am available in different pockets of time, depending on the day. I do have Thursday off entirely and greatly look forward to a second interview."
"I can make myself available, given a bit of notice. If it is an in-person interview, I'd need to take a half day, so would need a day to put in that request. I'm eager to move on to the next round, so please let me know as soon as possible so that I can make it happen."
"Since we likely work the same hours, I would need to take a half day off, and find a substitute. Can we set a date now so that I can get started on the requested time off?"