As far as gameplay ideas go, might I suggest looking at older games that were similar in play-style to the game you're making? The thing is, older games had to use simpler concepts which in turn are easier to recreate in SGDK2. Take the old 'dissappearing blocks over a chasm' trick. You watch the pattern, then try to jump across the blocks in time with the vanishing/reappearing. Simple to do, but was pretty interesting in the Megaman games.
For something a bit newer feeling, how about a hookshot (NOT grappling hook. Think Legend of Zelda) that would pull the character toward certain blocks. It could make for interesting tower climbs inside your huge castle... Crumbling tower pieces you have to hookshot from, perhaps?
Lastly, for something really uniqe, how about creating a layer on the maps that has standard tiles obscuring hidden doors, power-ups, etc. And the character could stand still and push a button to make that layer dissappear until you start moving again? It would be like the Lens of Truth (Ocarina of Time), and you could call it "Eye of the Phantom" or something. Imagine running into a dead-end early on in the game and then coming back with the Phantom Eye later only to find out it wasn't a dead-end after all and it leadis to treasure of some sort.
