If you've played Monument Valley, you have to play Forgotten Shores. The additional levels are multi-parted and have more complicated puzzles than you saw in the first game, and the entire expansion will give you around two hours' worth of casual playtime.
If you haven't played Monument Valley, then you should download both--the first game gives you the physics and architectural groundwork to fully appreciate Forgotten Shores' complexity (also, I don't think you can download Forgotten Shores sans Monument Valley).
For a combined price of $6, the Monument Valley collection is definitely worth it.
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