Stack'd is always a solid choice if you're going out to Walnut St. Their burgers are great and I love the fact you can customize what you want on your burger. Their house sauce is delicious and I always get that on my burger.
While you can drop a lot of money at Stack'd, there are certain nights where you can get good deals on items, like half-off burger nights. I especially love the $1 drink nights on Thursdays where you only have to spend a few bucks to have a fun night out. They make their drinks pretty strong so you'll get the bang out of your buck.
Stackd's milkshakes are pretty good as well and offer a lot of good, basic flavors like chocolate, vanilla, peanut butter, etc. Their fishbowls are a fantastic deal and are only $15 and even cheaper if you split the cost with friends.
Stack'd is my favorite place to go on Walnut st. and is a Pittsburgh classic for college students.