Some great ideas & tips in the thread that Cathy linked. Have you considered buying a tiramisu from a bakery (I bet the $35 one from Pasticcieria Natalina mentioned in the linked thread beats the pants off of just about any restaurant tiramisu out there), picking up a $40 bottle of champagne from Binny's (which would probably run you $120 at a nice restaurant)?
Clean up the apartment (this step is key, as piles of mail and a basket of unfolded laundry are not conducive to romance), dress up all fancy, dim the lights, light some candles, put on some music, and have a romantic night in for $75 (not counting dinner, which you planned on making anyway)...can't beat that