This one time, at math camp, @colinmac10 showed us how to use Socrative for exit tickets (and more!) if most students have a smartphone or laptop with them.

The Exit Ticket option will ask 4 questions. While students answer the first 3, you have time to create a question on the whiteboard. Very little advance prep is required and students can do the entire thing in about 2 min + time to solve the last problem.