Version tested: iPhone
In a world where £200 consoles are often packaged up without a bundled video cable, and chunks of games that we used to get for free are now sold to us separately, it's rather gratifying to load up one of your favourite mobile releases and discover that the developers have bolted 32 more levels onto it for free.
Like a lot of freemium add-ons, Whale Trail's Challenge Pack invites you to spend a token amount of money (£1.49) to unlock all its levels, but persistent players who have spent a few months plugging away at the main game should be able to earn them without handing over anything more than the 69p we paid for the game in the first place.
The original Whale Trail was another one-button phenomenon. We took control of a flying whale (obviously), tapped the screen to propel it upwards and released our grip to let it descend. The idea was to dodge through networks of dark clouds and collect a magical rainbow stream of bubbles that fuelled your flight, and the goal was to set high scores by going as far as possible. Like Tiny Wings, Canabalt and plenty of others before it, we chased that goal with every unlikely pound of flying mammalian blubber we could muster.
The Willow's Playground Challenge Pack offers a series of bite-sized alternatives to the main 'endless' mode, inviting you to swoop through tricky networks of clouds, gather tight-fisted sequences of bubbles and use your practiced thumb or finger to adjust your whale's arcing parabola as economically as possible to reach the finish line. If you do so with enough points then you can unlock up to three stars per stage, and as your stars total inches upwards through repeated play, so you unlock more stages.
Whereas the main game often had you glancing nervously at your bubble meter to make sure you weren't about to plummet to ground, that's less of an issue in the Challenge Pack. Instead you fixate on your score, often having to carve perfect arcs through the sky gathering every bubble and avoiding every cloud to make sure you secure all three stars. Over and over you plug yourself into the individual stages, refusing to settle for less, whipping out your iPhone or iPad whenever the spectre of boredom appears on the horizon and beating it away with a rainbow-fuelled flying whale.
The shift in emphasis is enough to transform Whale Trail into a new obsession again. After fighting your Game Center friends for dominance of the skies, now you battle your own reflexes to keep unlocking new content. Eventually you may give up and ponder that £1.49 lazy-unlock fee, but probably not, because even the most cack-flippered flying whale should find plenty to unlock and enjoy here before running out of bubbles.
App of the Day highlights interesting games we're playing on the Android, iPad, iPhone and Windows Phone 7 mobile platforms, including post-release updates. If you want to see a particular app featured, drop us a line or suggest it in the comments.