Welcome Diner is a fun little place right on the way out of Downtown, and very near the campus.
Their passion for food is obvious, they take their menu and ingredients very seriously, challenging themselves to make new and delicious flavours which rotate regularly. Their options seem somewhat limited on their one-page menu, but if you take a moment to read it over, you'll see vegan options (which, and I'm amazed to even consider saying this, are all delicious), gluten-free, and interesting sides.
The more cost-conscious diner might find the place a bit spendy (most menu items are served a la carte) but for its quality and location, the prices are more than reasonable. They have a full liquor menu, with creative drinks and twists on classics, and are open until 2 A.M., which makes it a great place to end your trip on a night out, but they are also open for breakfast at 9 A.M.
The atmosphere may put some off, as it is unapologetically hipster, with the staff and managers all dressed in their street clothes, and most sporting beards. It can make telling the crew from the customers slightly challenging, but it also affords a very casual and homestyle attitude to the restaurant.
All in all, the Welcome Diner is a great addition to the Tucson scene, which gives an unpretentious venue to experience cutting-edge cuisine and locally-sourced produce. I heartily recommend it to the diner who appreciates a fun experience and doesn't worry about formality, but who craves great modern food made by people who love what they do.