Mostly I modify the following as n Page Seo to rank well on search engines.

1. Content Modification

2. Meta Tags

3. Image Alt Tags

4. Bread crumbs

5. H1 to H6 Tags

6. Interlinking all the pages

7. Sitemap

8. W3C Markup Validation

9. Even Some time I help the designers to organize the stuff (content, images, links, menus etc)
on the pages to help User usability Experience.