I chose the Fire Domain as the Nature Bond, partly for thematic reasons, but also partly because I wanted some blasting (because aside from Flame Strike, Druids can't do crap for blasting), and stuff like Fireball is great for that; I'm our only full spellcaster, meaning I'm the only one with access to 3rd level spells currently, so having to fill the shoes of both a Divine and an Arcane full spellcaster is a very arduous task.
If you are worried about conserving spells over long dungeons blasting is an ATROCIOUS route to take. If you can ask your dm to undo this choice do it or ask about retraining it. Even if you want to go with a domain instead of a companion (which given all your cocerns you shouldn't) Fireball is literally the least efficient thing you can do with a 3rd level spell if you are worried about resources. Particularly if you are the only caster, your spells need to have more impact then that.
I considered an animal companion, but to be honest it's a lot of work, I'm not particularly familiar with their rules, and on its surface, they fall off very fast. If anything, right now they're at their strongest, but in the coming levels, they will become extremely weak, and really only serve the same role as a summoned creature.
This is rather inaccurate. Animal companions with proper equipment (at low levels this is barding, at higher levels it means an amulet of mighty fists and magic barding) hold up just fine so long as you pick the right one. In particular, the Big cat or the Deinonychus are quite effective.
Remember when you get higher level you will have more spells to buff the companion (and yourself). Those buffs are dramatically more efficient and resource saving then blasting spells. Up and until levels 14-16, the right animal companion with the right equipment and feats is not weak. Its quite good actually.
That tactic would work with stupid or animal-like enemies, but smart enemies would realize how pointless and weak they are and simply ignore them after maybe the first round. Sure, flanking is nice, that's why I try to set them up to get a Pouncing Flank, the burst damage on that, presuming some decent rolls, is pretty high. In our hardest encounter, I actually did ~70 points of damage in total between my summoned creatures in a single round on one enemy, and came very close to killing it outright.
So first of all, even if they aren't a massive threat, just being physically there in the opponents way is important. Even if the enemy attacks them once, and then they are just an obstacle and flanking buddy, that is a good use of a spell slot. Partiuclarly if with things like flanking bonuses of their own they actually get a few hits in. With just augment summoning, they are serviceable at least until you get to mid levels where you have more spells per day to work with.
My physical statistics aren't that great since I actually originally planned to build a sort of summoner type of character, but didn't realize that SNA is a crappy, ghetto version of SM. I am at a 16 Strength (it's 14, but with ABP at a rate of Level + 1, I get the +2 Enhancement bonus). With a Medium-sized Wildshape, that becomes an 18 Strength. I also acquired the Barbarian VMC and am able to apply the UCBarbarian Rage (which at this point in time is actually better for me), granting an extra +2 to attack and damage for those moments when casting spells is useless or isn't necessary, Elemental Assault through extra Race Points allows me to add 1D6 Acid/Cold/Electricity/Fire damage to all of my attacks, and I use Power Attack, which is at a -2/+4 rate [we are actually 6th level, I took a dip in UCMonk for better Wildshape survivability by adding my Wisdom bonus of +5 to my AC at all times over my crappy armor].
Unfortunately, my to-hit actually sucks, and as much as I'd like to remove Power Attack, I need the damage it provides, especially against enemies who have DR. Presuming my Deinonychus form, I have 5 attacks, with 3 Primaries. My total BAB is 4 Base + 4 Strength + 2 Rage + 1 Enhancement - 2 Power Attack = +9 to hit for my primaries, +4 to hit with my secondaries. Most enemies at 6th level that we've faced so far usually have ~24 AC, which is difficult for me to hit (and all of us, except our tank, is 3/4 BAB).
Presuming I hit, I have 1D3/1D6/1D8, depending on method of attack (1.5, 3.5, or 4.5), +4/2 Strength, +2 Rage, +4/2 Power Attack, +1 Enhancement, and 1D6 (3.5) Elemental Assault. This totals in an average output of 18/19 damage per Primary attack hit, or 12 damage per Secondary attack hit. Presuming they all hit (highly unlikely, but I've come close before), I'm dealing ~ 80 points of damage per round, which is pretty ridiculous considering that I don't have an optimized Strength, nor do I have access to Dragon Ferocity and Feral Combat Training, which would allow me to apply 1.5x Strength to my Natural Attacks (a plan that I just might pursue later down the road). Enemies with DR 10 or Resistances (which seem to be becoming commonplace) reduce that damage to 8/9 or 2, perhaps even less than that, which isn't much, even when they have average hit dice (and we have 3/4 Hit Dice).
The problem I have with Wild Shape as of right now, is that I only have one use of it. The duration is decent and good to last over the course of a few encounters, but it's not all-day, and I don't know how long we're going to be in this dungeon (if we plan to rest, the 5 hours that I have with Wild Shape won't last long enough to be relevant if we're ambushed), so I'd rather not waste it during the day, even if we could use it, when we're ambushed with low HP, when we'd need it.
So first of all, I understand wanting big damage numbers, but if you are worried about hitting don't always use power attack. If you are facing opponents with consistently high AC (24 is high for 5th level characters) STOP USING IT. More damage doesn't mean anything if you miss all the time. And remember this should only be PART of your contribution. If you do 50-60 damage and your companion does 40-50 damage, that adds up.
And while 5 hours isn't forever its a long time. Its more then 1/3 of your adventuring day and should cover at least a couple encounters. And for the others, you can use more of your available spells. But wild shaping as your first action of the first serious seeming fight of the day is a very good idea most of the time. Even if there are lots of encounters, it usually doesn't require more then 8 hours of in game time. And if the encounters are spaced out by the miles required to make a longer day feasible (meaning hours of travel between encounters) then you can probably rest at some point.