The easy way would be: in the menu add a special little button that shows a code like "12345", a unique code for each game. Then user can use this code to get bonuses in the other games by typing it in some text box.
Or you can add links to your other games and just give out rewards as soon as user opens the link. If you specifically ask them to install the game in exchange for reward - most users will actually do it, because they don't know that you cannot detect the install. That's how it always worked with requesting a review/rating.