I'm looking for good choices for a Druid who relies on casting spells and avoids melee combat whenever possible.
Let's start with some Classics:
-Improved Initiative
-Spell Focus Conjuration/ Augment Summoning
-Natural Spell
-Extend Spell
-Toughness(?)
Are any of the non-Core feats tasty? Superior Summons looks nice, what else do we have that's of interest?
Scribe Scroll!
Partly it depends on your playstyle. If you cast a lot of long-term buffs, Extend Spell can be very useful. If you prefer throwing spells with a save requirement, then (Greater) Spell Focus may be worthwhile if you plan to get most of them from one school. If you use a lot of rays (though that's more a wizard thing), then Weapon Focus (ray).
Quicken Spell is very handy at high levels (probably not before 11th level).
Various metamagic feats might suit your style -- there are too many of them to really list.
What level?
Different feats apply for different types of spellcasters.
The feats you listed (except "toughness") are typical druid summoner feats.
Druids have other spells too and are an effective battlefield controller, so you could instead focus on improving your saves for specific magical schools. That route would probably be spell focus: XXXX and greater spell focus: XXXX, along with combat casting and metamagic feats like "selective spell" so you can target enemies and leave your party out of the effect.
Direct damage is also possible, which would mean different schools to focus on but the same basic idea applies. Metamagic feats like "empower" or "maximize" are very important for that role.
Druids have a limited number of buffing spells, but what they have are pretty nice. Usually in combat buffing becomes an action economy net loss though, so many buffing specialists also end up in melee or at range themselves so they benefit from many of the same feats as the direct damage druid.
Finally there is probably the least common approach (which is the approach I took with my druid) of the archery druid, which means you have to get the basic archery feats (PBS, precise shot, rapid shot, etc...)
To answer what sort of feats to take, it would help to know what your fundamental preference for playing the druid is.
I am using the Elemental Focus for +1 DC on Electricity Spells. Spell Focus is probably better but i took it flavor wise.
Combat casting seemed classic and good to have.
I built an 8th level sorcerer for a one-shot session and took spell focus, greater spell focus and elemental focus on an air elemental bloodline, and added a headband of alluring charisma, and his DCs were crazy high. He also took "empower spell" and "magical lineage: fireball." He was quite effective in the role of ranged artillery.
Combat Casting is kind of party/DM-dependent.
If your party is pretty good about keeping you away from the baddies, or your DM lets monsters ride the fighter all fight, then you may not need it.
My rather bloodthirsty, melee oriented party is great at keeping baddies away from me, especially when I'm flying safely above the fray in air elemental or bird form. Point taken, and I agree with the rest of your post (my scouting abilities and Disable Device skill makes me the de facto party Rogue). Superior Summons gives me one more shield against enemies, so I think that goes on my must have list of feats. If only there were feats to help against damage reduction for my summoned critters.
And yes, Scribe Scroll frees up precious spell slots.
To answer what sort of feats to take, it would help to know what your fundamental preference for playing the druid is.
Think of a flying, spell casting, sneaky, kinda cowardly type of Druid who's disappointed if he takes much (any?) damage or has spells unused at the end of any given day. [Edit to add: We started at third and plan to go to about 16th level. At 9th currently. But though I'm looking at this mainly from my current character perspective, I'm also interested in general terms.]
Thanks all for the responses so far.