How can I increase traffic on a website without spending money?

I only offer organic ways increasing your SEO:

Simplest way to take your under-performing pages to the top is to use good SEO tool. Lot’s of tools offer freemiums that are basicaly just abridged versions of tools to give you the taste of full version, but if you know how to use it, you can do a decent research even for free. I’ll show you how can do it with Serpstat. This tool allows 30 queries with limited amount or reports per day for free which can be enough. I’ll give instructions on 3 highly resultative techniques:

First, enter researched domain in the tool (nike.com, for example), select database (Google US, for example), click on “Search”, and in “SEO Research” section click on “Tree View”. Now you have full list of URLs and keywords for which these URLs are ranked top-100:

Click on “Filters” and set “domain position for a keyword” to show results from 11 to 20 (which is a 2nd page of Google results):

You’ll get the list of URLs with great potential to be pushed to the top of search. The Pos. metrics show the position of page in SERP for given keyword and its dynamics. Now set up internal linking for these pages using these keywords in anchors. “Tree View” saves your time tremendously in finding pages to be pushed to the top while cutting manual labor.

• 2) Cluster Research. Enter your keyword to Serpstat and go to “Keyword Research”> “SEO Research” then click on “Cluster Research”.

You’ll have the list of all search queries related to the your keyword. What you’re looking at is a complete list of keywords with which you can optimize your pages.
The uniqueness of this feature is that you won’t always get those keywords from Google Keyword Planner. Chances are that your competitors don’t have them.
Note the “Connection Strength” metric. Its max. value set to 20, i.e., it measures how many sites from top 20 of search are ranked for given keywords.

• 3) Search Suggestions. Enables you to find all search queries related to your keyword. Best place to start generating ideas for traffic-driving content. Enter your keyword and in “Keyword Research” section of the tool go to “SEO Research”>”Search Suggestions”:

You’ll have autocomplete suggestions from Google. Now click on “Only Question” filter:

You’ll get questions from Google’s autocomplete suggestions. Now you can easily spot trends and get the questions that users ask themselves. All you have to do is to give answers to put your article into limelight.
Effectively, you would set a task to your copywriters to make content using this keywords. Then you’ll observe how this content rise to the top.