There's a nice feat in Varisia Player's Companion, Deadly Dealer that uses arcane strike with cards.
Yea, gambit.
Yea, just mentioning it to the people wondering about Gambit :)
Summon Instrument and beat them over the head with it. El KABONG !
A double-bass ? (Definitely two-handed)
Adamantine Double-Bass aren't exactly typical of their types.
The important part really is the word "typical". Even a bell, mostly made of metal, wouldn't typically be adamantine. It's the same thing that stops you from summoning Drums of Earthquake when you summon a drum.
Steel guitar?
Some handbells get pretty big. I can't find a picture of it, but from memory, the larger bells are rung two-handed (although by moving the clapper, maybe).
http://www.sawlady.com/photos/CroslandMoorHandBellRingersUKyorkshire.jpg
However, you may want to use cymbals instead. It seems reasonable that you'd get a pair of them -- maybe you could TWF -- and also, possibly, have the potential to deal slashing damage.
Other than that, resonator guitars are made of metal although entirely out of the time period. Autoharps are wooden, smaller, but really heavy and solid-feeling for their size.
I, however, would beat people to death with bagpipes. It makes no sense, sure, but it would be hilarious. Imagine the squawks the poor things'd make.
Largest brass instrument is probably a tuba ... however timpani (aka kettle drum) are heavier and typically made of brass as well.
If you are a larger creature, yes.
A medium sized creature can't summon an instrument for a Large or larger creature though. You, a medium creature, can still summon an instrument, even though a Diminutive creature couldn't hold that instrument in two hands. It's relative to your size.
The implication is that those are your hands. Otherwise, things get stupid silly.
"But Mr. GMMMMMmmm, a god could hold this organ the size of a planet! The rules don't say they have to be your hands, even though they only work for you."
Using Summon Instrument and improvised weapons as a way to always have a weapon is a hilariously clever thing though. Especially with no Material Components!
The Traveler's Any-Tool from UE might help too. You could make a "weapon" out of whatever is necessary. The spell Solid Note would work too, maybe. I'd have to read it, but it's basically "Summon Not-Quite-Immovable-But-Fully-Shapeable Rod."
With creativity, you could do a lot with that. It could conceivably do any of the physical damage types.
Oooooh. Maybe. The only thing stopping me from saying "yea, that'd work" is the handheld thing. It's a strange enough corner case though that it may just be best to assume no, or ask your GM.
Are there instruments made mostly of metal?
Get your hands on a gong.