The Core

If you can code and can be arsed (which I assume is the case if you want to avoid plugins) I believe you can write a nice function that does that depending on the category. If not then I think there actually are plugins for this.

I think you could do something at the theme level using the Rewrite API. Since it's an API, it's not likely to break as readily as a plugin with a full interface and everything. Should be a an easy little script addition.

http://codex.wordpress.org/Function_Reference/WP_Rewrite

Can't you just manually tweak the .htaccess file and make this happen?  

.htaccess doesn't create URLs, it just acts as a traffic cop for incoming URLs. The problem is that WP gives very little flexibility over URL patterns.

You can do %category%/%postname% but you can't do any if/then logic. What gimpie needs is

if %category% == 'news'
    pattern = /%category%/%monthnum%-%day%-%year%/

elseif %category == 'reviews'
    pattern = %category%/%postname%.htm

I gotta ask, why .htm? Do you have legacy URLs to deal with?

I would go with
 - extensionless URLs
 - yyyy-mm-dd dates or, distant second choice, dd-mm-yyyy. The former simplifies sorting if you ever need to output and sort URLs by date.