Malec2b, creator of Dadaists Gone Wild, has some other nifty games tucked away on his site. Today I'll point you to two of his shmups that are similar to Cactus' only by virtue of their uniqueness. Optimization is the common thread between these two small Game Maker games, so puzzle elements play a part in each. They aren't incredibly mind-blowing by any means, but they've got some cool ideas driving them. If you're a fan of either the shooter or puzzle genre you should take for a spin.
Turn Based Shmup (TBShump?) is a mashup of two seemingly incongruent genres; one rewards twitch gameplay while the other values tactics. From either viewpoint the game's pretty basic, as you have only two enemies and three verbs (move horizontally, fire, or wait). Red enemies move ever downward in a straight line and yellow UFOs hum across the screen and fire like they're from Space Invaders. The catch is that you have a finite amount of fuel; every action you take whittles your supply away. When you kill an enemy they drop fuel, but you have to retrieve it yourself. It's set up so that if you only shoot the easy red dudes you won't last that long; you'll have to go after the harder-to-nab UFOs to stay afloat. Despite its simplicity I enjoyed divining an optimal play order, but this is coming from someone who dropped literally hundreds of hours between Tactics Ogre and Final Fantasy Tactics.
Any Enemy You Like is a straight shmup, but that doesn't make it any less odd. The pacing and intensity rests solely on your shoulders, as you choose what enemies to face and when they appear. Your foes range from standard cannon fodder to a full-on boss, and are activated once you press the corresponding number key. You can summon any number of enemies, with the only rule being you can't pick the same type twice in a row. Time is the optimization crux here, as you have only a minute to amass as many points as possible. Whether you want to play a bullet-hell, a boss-centric, or laid back game is totally up to you.
Both games have some cool ideas, and could easily be extrapolated onto larger projects. More complex shmup systems would be the obvious choice, but these approaches could be used on other genres. I'm thinking of a platformer that has a large verbset and a similar action system to TBShmup, where the puzzle is to simply get to the exit. I know I mention Spelunky on a seemingly-daily basis, but imagine a Roguelike that takes place in a similar vertical 2D playing field but is turn-based in the traditional Roguelike manner. Both games would obviously need to have deep gameplay to be compelling, but they sound pretty cool. And if Any Enemy You Like was a platformer, it'd have multiple levels of challenge to it (spikes, baddies, etc.) and you would choose which elements to face at the outset. Play these games and see if they ignite the idea-center of your brain like they did for me.