Druids have Dehydrate as a 3rd level spell, 1d6 Con damage +1 per 3 caster levels, max of 5
This ability would make that 2d6+10 Con damage at level 15.
That poison alone is enough to drop a storm giant with a single application. I'm assuming since this is a poison, she could pass this around to the entire party?
Arc of Lightning as a poison could also be fun... connect a line between the creature you stab and something else in Close range and deal 1d6/level electricity damage to everything between those creatures.
Creeping Cold also comes to mind. It allows a Fortitude save but only for partial effect. Total it would add up to 12d6 of cold damage if they fail both saves, or 6d6 if they save both times.
Saltray would be similar... at level 10 it caps at 5d6 damage, which the target takes automatically, with a Fortitude save to see if you are stunned for 1 round or not.
Moon Bolt too... 1d4 Str damage per 3/CL's, Fort save for half... or makes an undead helpless for 1d4 rounds, Will negates... or you could hit an undead with a Spark of Life poison and get rid of their enormous immunity list, which includes poisons.
Honestly above examples are what I'd be most worried about. Since the text says "as the spell", even if they save vs the poison they still take the partial effect... so in the case of Heartfire concealment is negated for 1 round/level and they take half of 1d4 fire damage each round.
Normally this isn't a concern but now the entire party can fire this stuff off on multiple targets or stack it all on one target.
Also since it says "must include a save" instead of something sensible like "must include a non-harmless save" I imagine there's any number of hypodermic buffs you could create with this ability.
EDIT: If you want to get books thrown at you, then consider that the ability states that it must be one spell that you know... it doesn't specify that is has to be from the druid spell list. This occurred to me when I wondered if you could apply divine metamagic to the spell when brewing the poison.