LJWorld did a nice write-up of birthday freebies. By far my favorite Lawrence birthday freebie is going to Liberty Hall to see a movie for free, plus free popcorn and soda, plus a free movie rental.
A couple could easily have a night out for a birthday in Lawrence for $20 (buy one dinner and one full price movie ticket), perhaps as little as $6 (share the free dinner, and buy one student rate movie ticket.)
Here are a few more birthday freebies not on the LJWorld list (most require you join their email list, so click the links!):
Free cookie dough from Papa Johns
Free milkshake at Arbys
Free bagel and schmear with drink purchase at Einstein Bros.
Free burger at Dempsey's Burger Pub
$10.00 off coupon at World Market
UPDATE: I updated this 7/19/12 with some new links, etc. Click here for the new entry: Birthday Freebies Update