I would use your 'Drain mana to do something really out there. I think that Oath and Landstill don't sound casual enough for your crowd. Dark Depths seems like a good suggestion, though. Drawing upon your inner Timmy-Johnny is your best bet, in my opinion. Reach for Tooth and Nail, Biorhythm, Time Stretch, Earthquake, Hurricane, Djinn Illuminatus, or Tidespout Tyrant. Use the mana to cast a bunch of little spells to empower Wee Dragonauts or Gelectrode. Funnel it into a Gemstone Array and find something to do with it later. The problem is honestly too broad to offer you a really good solution. Suggest two or three other cards that you'd especially like to use in this deck and we can start hammering out a list for you.
That's the spirit, and the response I was looking for! I really want a two-colored deck, but I'm worried about the mana--getting UU consistently for drain and mana for whatever other color I use. I do have 6 blue fetches (4 deltas, 2 strands) and 6 blue duals (2 sea, 2 trop, 2 volc)...so I guess I could go UB, UG or UR pretty easily.
Tooth and Nail sounds really cool, though. What do you think would be the best pair of creatures to pull out? I really like Platinum Angel and Leonin Abunas. Sundering Titan is similarly fun. Maybe I could just throw in a couple of other big artifacts and throw some Welders and TfK's in, too!
Welder Tooth
//Artifact--6
1x Duplicant
2x Mindslaver
1x Pentavus
1x Sundering Titan
1x Triskelion
//Blue--16
4x Brainstorm
4x Force of Will
4x Mana Drain
4x Thirst for Knowledge
//Green--3
3x Tooth and Nail
//Red--3
3x Goblin Welder
//White--2
1x Akroma, Angel of Wrath
1x Leonin Abunas
//Mana--30
2x Izzet Signet
4x Pentad Prism
2x Simic Signet
2x Flooded Strand
12x Island
4x Polluted Delta
2x Tropical Island
2x Volcanic Island
Or do you think duals, fetches and forces push it into non-casual territory?