Seriously... you are a Druid 20. You have
shapechange. Wild Shape is great, but
shapechange is better: it's the ultimate all-round buffing, movement enhancing, offensive, defensive, utility spell. With a Rod of Greater Extend Spell, one casting lasts for 6 hours and 40 minutes - and that's without resorting to a caster-level boosting item of some sort. During that time, you can change form once per round as a free action. You get all extraordinary and supernatural abilities of the form. If you change into a form that could normally hold your gear, you're holding (and/or wearing) it. If your current form can't hold or wear the equipment, it melds into your form until you turn into something that can.
This means you don't have to travel with a dressing room and attendants, which is a good thing.
I admit this doesn't make the point about Druids having Real Ultimate Power - any class with the spell could do the same things - but if your only concern is beating the stuffing out of level-appropriate challenges, Shapechange is hard to top. The only drawback it has is that you can't use Natural Spell to cast in forms that wouldn't normally allow you to throw spells around... but there's nothing stopping you from Wild Shaping before you Shapechange if there are benefits in doing so (a form with mighty escaping powers such as an Air Elemental is good for when your Shapechange gets dispelled).
Stack long-duration buffs on top of what you can pull out of the Monster Manual and you're golden. If you can buff your caster level by 2, you can walk around all damn day as a Solar and not even care: your magical gear will mostly resize to fit your new form so, as per the FAQ, you get to keep it. Your preferred weapons won't resize (although of course there's nothing preventing you from carrying a Large weapon around for just such a contingency), but it's a great form to start a combat in. Or, you know, be a Nightwalker and summon dread wraiths to hang out with you. Then be a
different Nightwalker and summon
more dread wraiths to hang out with you. They'll hang around for an hour and you can just keep doing it, so why not? (Admittedly, I can see a DM not allowing this for sanity reasons but I can't see why not by RAW)
You don't even need to waste spells on healing: if the fight gets a bit tricky, turn into something that can be ethereal like a Nightmare (or Cauchemar), leave for a few rounds and then turn into something that has "Lay on Hands" as a supernatural ability. Or decent regeneration if you can spare a bit longer. Guardinals are good - they can heal their full normal hit points with their paladinesque healy goodness.
And remember: you can share spells with your animal companion. Including
shapechange. OK, it has to remain within 5 feet of you once cast, but two asswhuppin' machines for the price of one is not to be sneezed at.
Or... be a Balor. A raft of immunities and
true seeing continuously? Yes please. But you have an animal companion, so be TWO Balors. Strictly speaking, you even get the
+1 vorpal longsword, on the grounds that it's listed as a supernatural ability. Pit Fiends are good melee types, too. For immunities to all kinds of nastiness, it's hard to beat constructs, so keep golems in mind if you're facing something throwing save-or-lose spells around.
I can't add much to what Dandu said about spells, but don't forget the oft-overlooked
antilife shell for hedging out all living creatures. Pick up a
liveoak Treant guardian as a speedbump: it's not the greatest combatant, but being plant-based - along with shamblers - it's practically immune to a lot of the scarier powers of high-level undead and the like.
Harking back to the Rod of Greater Extend Spell, an
elemental swarm lasts as long as
shapechange: elementals are immune to all kinds of stuff, and having 2d4 Large, 1d4 Huge and 1 Greater Elementals along as backing singers is going to slow down most creatures.
If something's corporeal and can't fly,
reverse gravity is a good way to spoil its day. Although damn I miss the 2nd Edition version!
Stoneskin is another buff that can be extended for nigh-seven-hour goodness.
Freedom of movement, ditto. Hell, so is
foresight, although you probably have other things to do with 9th-level spells.
Death ward doesn't last long, but unless you get yourself an item to duplicate the effect you probably don't want to leave home without it memorised.
There's also
awaken, of course. However I'm no expert on this and I suspect that without non-Core feats and spells you're not going to be able to get the best out of it. It's a pity you didn't bargain for the Spell Compendium if nothing else: that would have made this exercise easier. Still, provided your son is sticking to core as well you ought to be able to show him a few things. Is using
transport via plants or
tree stride on your Awakened tree or
liveoak ally too weird?
For melee hideousness and just-add-magic speedbumps, keep a few bugs in a jar: a 20th level caster can create 1-3 Colossal vermin with a single 5th-level spell, and those are handy against melee threats. A Colossal Scorpion has a +58 grapple modifier: I'm just saying, is all.
Finally... study. At the very least, learn the different creature types and what they're generally immune or vulnerable to. If your son is following the same restrictions you are, you should be able to gen up on all the CR 15+ monsters in MM I - not counting templated creatures or those with class levels, there are only 16 of them after all.
See if you can talk your son into using one or more of Spell Compendium, Complete Divine, Masters of the Wild or Savage Species. Also find out if he's prepared to let you take monstrous feats from MM I due to spending so much time in wild shape. If he agrees, come back and let us know
