My own party is 10th level as well. Though, they are more combat heavy (barbarian and a fighter) so my problems may be less severe.
I'm like you with regard to imposing artificial hindrances in the game. I would never stoop to lining the walls with adamantine, or placing in arbitrary anti-magic zones unless there was a specific reason to do so.
However, in general, these are the things that I find make an enjoyable high level game -
- don't assume that a single BBEG is going to be any challenge for a party of 10th level PCs. The trick is to have the enemy attack in waves. My party is clever in that the fighters are always up front dealing with the bad guys while the rest stay in the back and offer secondary support through spells, or ranged attacks. Their weakness comes when I hit them from behind, I can do massive damage to the rogue and spellcasters before the party knows what has hit them.
- exploit their weaknesses. The barbarian and the fighter in my groups are complete and utter bad asses. But their will saves suck. One successful domination (and with a follow-up save to convince them to attack their own party), and you've got an entirely different encounter on your hands. Enraged barbarians also attack anyone that attacks them as well.
- let the PCs be their own worst enemy. My sorcerer felt real cocky, and went off exploring by himself in a submerged section of dungeon. He was doing ok, using Blink to breath while ethereal - until an Aboleth dominated him, and he drowned once his blink wore off.
- Place the party in a "living" environment. Like others have stated, the inhabitants of this place aren't going to like being invaded. Plus, digging big holes and fighting purple worms are going to create a huge amount of noise - especially in a dungeon. Think about Moria where Pip (or was it Meri?) drops the chain down the well in the dwarven tomb. He awoke the entire dungeon and alerted everyone to their presence.
- Define tactics for all creatures. My big weakness is forgetting about all the abilities that a creature has. I got my last BBEG killed because I failed to remember that his helm could cast d-door 2/per day. Had I remembered that, I could have had him escape to fight another day. So, always create a tactics outline for each monster, especially the important ones. Spellcasters will often have active spells up, especially ones that have a duration of 24 hours, so it's important to note those as well. The tactics that you define don't have to be elaborate, but it should start with all the most deadly abilities first (i.e., the dragon will begin by casting dispel magic, and on the following round it will breathe fire), etc. If you just plop a dragon into an encounter, you might not have time to think about what it's best options are, or even what spells it currently has memorized. A monster can easily jump 3 or 4 CR levels if you just plan accordingly. Also, include a plan for failure. Like "if the dragon is being defeated, he will attempt to cast teleport to flee into the forest", etc.