By guerilla testing I mean brief, informal testing performed on random people who happen to be about. Here, I'm specifically talking about testing on strangers in public place, not people within your organisation or family and friends.
Use giftcards for an establishment close to your testing area. Once for guerrilla testing, I set up a laptop just outside of a Starbucks near our office for testing. We had a lot of people agree to do the test for a $5 giftcard, I'm assuming, because they could use it right there and then. The test was very brief (8 tasks) that took about 10 minutes to complete. We had ten giftcards and they were all gone in about two hours.