In the old days, you built a stronghold and raised an army. You want a 100-strong cavalry? Then besides 100 cavalrymen (who will want pay), you also need 100 warhorses, stables (lots of them), stableboys, and lots of food to feed everyone. You might also want barding (horse armor). Oh, and your cavalrymen will want armor too. Are you cheap and only buy them padded leather? Or do you go all-out and outfit them with scale mail? Or even plate mail? They'll want weapons too. Spears or lances, probably. Plus swords, if they get knocked off their horse.
And if you have a cavalry, why not an infantry too? Maybe 200 pikemen? And 40 crossbowmen to go along with them?
Just some ideas.
Matt Colville's book. It's pretty awesome.Which reminds me, there is a truly EXCELLENT supplement on DMGuild, Strongholds & Followers, something like that. It is a whole book just for building castles/forts per class, and acquiring henchmen and hirelings with stats. There's also a built in monster manual and an adventure. It's a fantastic sourcebook. You could just spend your mid to later levels doing castle stuff if you wanted.
Bling. My characters often buy and keep jewelry and art objects we find, as well as gems.
Also, you can buy an inn to get local gossip (as originally happened with Lord Robilar), even if you run it at a loss.