Here are my recommendations of where to eat in no particular order:
Japanese
Izakaya Den
Cracking food, funky vibe, probably med-high end in terms of cost - not outrageous but 2 courses (read: 6 sharing plates) with a couple of beers and a cocktail is going to be $120 for two people.
Mexican
Mexicali Rose
This is just a short (5-10min) tram ride from the CBD but has exceptional mexican at very reasonable prices. I've never been disappointed here (have eaten there 10+ times regularly for birthdays etc). The combination plates are brilliant if you like different textures and mixing it up and if you have room the desserts are very good also (try and save room for the chocolate tacos, you'll thank me).
Modern Australian
Cumulus Inc
This is probably my favourite resturant in Melbourne at the moment, the qualtiy of food, service and atmosphere at this place is off the charts. It has an open kitchen which can be cool to watch if you're into food (you can ask to be seated at the bar looking in to the kitchen). If you want to totally wow your Mrs without breaking the bank this would be the place I'd choose. It is going to set you back a bit depending on what you order etc but I've always been happy with whatever the bill came to (and no I'm not minted). There's something for everybody on this menu. You may have to wait for a table depending on what time / night you go (we always try and get there early like 6 ish) and we've never waited more than 15 mins but you can just chill out and have a drink at their bar or upstairs while you wait.
Greek
Stalactites
If you fancy a bit of a meat fest this is definitely the place, you can get s humble souvlaki or getting a sharing plate with lamb skewers, mixed giros from the spit, lamb cutlets, hot sausages and greek rissoles for $20 per person. Quality food, always busy and quite loud so not really the romantic table for two type deal but very good if you feel like a filling meal.
I've tried to cover most of the bases there, if you're leaning one way or another or want more options let me know. I've included all the links to the restaurants on Urban Spoon so you can check them out yourself (can see most of the menus with prices etc). If you want cheap and cheerful type food let me know as there's heaps of them as well.
Cheers,
-Scott