Asklemmy

I have quite a few recommendations. They are not quite non-freemium since they are free, but if your reason to prefer non-freemium is to avoid ads and in app purchases, then none of these games have them. They are all also open source and the link I provided below will be for their respective F-Droid version (note: some of these games are not available on Google Play Store):

• Simon Tatham's Puzzle - A collection of many puzzle games ported to Android. The puzzles include Minesweeper, Net, Sudoku, and more.
• Simple Solitaire Collection - As the name says, it's a collection of card-based Solitaire games.
• Simple Brick Games - A game that imitates very basic handheld with very small pixel count. Games include tetris, breakout, and more.
• 1010 Klooni - A puzzle game where you have to use various shaped blocks to fill an entire horizontal or vertical line.
• Tower Jumper - This is a clone of a popular android game in which you move a jumping ball and try to reach the end of a tower avoiding the obstacles.
• Ball2Box - A puzzle game about shooting a ball into the box.
• Anuto - Another ugly tower defense.

This particular game requires far more attention and strategy than the ones listed above, but it can be played once every few minutes if you constantly save and reload (just don't play online):

• Unciv - 4X civilization-building game, similar to Sid Meier's Civilization series.