What is the best way to improve counter tops without completely replacing them?
As DA01 mentioned, there are epoxy-based methods for refinishing laminate counter tops in the $100-200 range. They seem to have fairly positive reviews:

Rust-Oleum Countertop Transformation Refinishing System 


$194.21
Covers 50 sq ft
(cheesy how-to video showing how 'ridiculously easy' it is)


Counter-Coat


$179.95  
Covers 35 sq ft 
(application instructions)


Giani Granite 


$79.40
Covers 35 sq ft
(equally cheesy how-to video showing how 'ridiculously easy' it is)


Note: I have not personally used any of these products, and I am in no way affiliated with them.