The onpage activity are :
1. title tag
2. url
3. robots
4.sitemap
5.content

Meta Tags
Meta Description
Body Tags (H1 to H6)
Keyword density
Image optimization
URL structure
Unique contend

1. Meta Tag creation
2. Content Optimization
3. Internal linking
4. Robots Text
5. Sitemap Creation

There are many On-Page important factors like !
Meta Tags (Title, Description etc..)
URL's
Website Navigation
User Interface
Content (Content is always King)
Keywords(Keyword is compulsory in content, title, description,alt tag)
Website Speed
Internal Links
Outbound Links
and many more..
Google has more than 200 factors for on page.

I think
* Heading Tags
* Keyword Density
* Meta Tags
* Addition of Images
* Content Interlinking
* Linking out to External Websites
* Write Engaging Content

On-page Activities are

Meta tags
Site maps
Robots
Content Optimization

1. Page Titles
2. Meta Descriptions
3. Meta Tags
4. URL Structure
5. Body Tags (H1, H2, H3, H4, etc.)
6. Keyword Density
7. Image SEO
8. Internal Linking

There are so many steps you need to follow, so you can read more on page blogs.

Meta Descriptions
Meta Tags
URL Structure
Body Tags (H1, H2, H3, H4, etc.)
Keyword Density
Image SEO
Internal Linking

1. Page Titles
2. Meta Descriptions
3. Meta Tags
4. URL Structure
5. Body Tags (H1, H2, H3, H4, etc.)
6. Keyword Density
7. Image SEO
8. Internal Linking

Meta Tags
URL Structure
Hash Tags (H1, H2, H3, H4, etc.)
Keyword Density
Image SEO
Internal Linking

11 Best On Page SEO Techniques 2017
①. Title
②. Meta Description
③. Post URL
④. Heading Tags
⑤. Keyword Analysis And Their Use
⑥. Keyword Density In Content
⑦. Alt Tags In Images
⑧. Internal Linking
⑨. External Linking
⑩. Use Of Videos In Content
⑪. Bold, Italic & Underline Tags

Optimize URL structure

Meta title and Meta description

Content optimization with fresh content and keyword density

Images with Alt Tags

Implement xml sitemap

H1, H2 tags

Interlinking

Open Graph tags

Social media sharing buttons

Create a 10X product or service,
Customer service
Focus on web searchers' intent
Have a responsive social media presence
etc......,,,,,,,,,,,

* Heading Tags
* Keyword Density
* Meta Tags
* Addition of Images
* Word Count Per Post