What are some nonmagical things to spend massive amounts of money on?

[Biggus]

Say you've got, like, 3 million gold (due to, for example, being created as an epic character). If your character has something against magic items, or if you're limited to only having 4 magic items totaling no more than a certain percentage of your wealth (as suggested by the DMG)... What on earth are you supposed to do with all the rest of that money?

Are you referring to the "limiting magic items" section on p.209 of the DMG? If so, I think you've misunderstood its intent. It's an abbreviated version of the sidebar at the bottom of p.23 of the ELH, which goes on to say that the purpose of those limitations is to avoid a new epic character spending all of their wealth on a few ultra-powerful items. As far as I can see, it's expected that you'll spend the rest of your wealth on other magic items, it's just that none of those other items can be worth more than 10% of your total wealth.

I know magic items are by far the most obvious (and most useful) choice, but apart from those, how would you maximize the effectiveness of this large fortune?

As well as just getting spells cast for you, there are a number of spells you can get made permanent on you.

Invest in a lot of diamonds; leave some of them at a major temple of your choice so you can always get a True Resurrection if needed (or your friends can get one for you). Also they're a way of carrying large amounts of money about which doesn't require you to have Strength 30 to avoid being encumbered.

Finally, in the ELH (p.114) it suggests that some temples will sell favors, which are notes which can be redeemed for spellcasting and cost between 1,000 and 100,000GP each.