Pre-theater (early - not going to theater) deals?
I would go to Ouest on the UWS for their early bird special. Atlantic Grill on the UES also has early specials which are good if you want to stay East.
Many restaurants have early bird specials, typically if you are seated before 6:30pm. You do not need to go to the theater district unless you want to.
You don't indicate any cuisine preference, so I'll take a stab...
Sel et Poivre, on Lex, b/t 64th & 65th, is a cozy, charming French bistro with very good food. They have a pre-theater prix-fixe (4 - 6:15 p.m.) for $19.95, and an all-evening prix-fixe for $27.95. The menu on their website appears to be the same for both. So, it's just a question of what time you get there.