CAGE STG is actually a pretty good vertical shooter created in the style of Hudson Soft's Star Soldier, and though the trial version only features three playable stages the game does not limit your playtime by any means. You even get to unlock additional characters to use if you manage to fulfill a condition or two by the end of the third node.

To upgrade your default weapon, simply grab the small blue gems that appear occasionally when you've vanquished an entire wave of enemies. (source, download page)