That first level dungeon needn't be a complete borefest.
Giant Spiders are apex predators, hiding in dark cracks until they pounce on you. They come with a Save or Die bite as standard.
Trolls regenerate from any amount of damage. Limbs can drag themselves back to torsos and reassemble. The reason they're so ugly is that you're not the first person to try burning them.
Orcs aren't much different from humans, physiologically. The scary part is that they're complete bastards and there are hundreds of them. The room with four orcs is a trap. The rest of the gang is waiting behind the door with firebombs and a complete lack of morals.
Goblins are innately magical and just want to screw with you. They'll turn your water into piss, wake up the sleeping dragon and pack your firewood full of exploding fairydust. If you chop one in half you'll spawn two smaller, squeakier goblins. If you set one on fire he'll turn into a mini-elemental. When they hide under a rock you'll lift it to find nothing but a stinkbomb. Just hope you never run into one of these creatures.
Giant Rats are completely filthy. Even if you don't get bitten you're not getting out of a giant rat nest without something. Ticks, fleas, general itchiness. Your digestive system won't be quite right for days afterwards. Oh, and remember how Orcs are never alone? If you see a lone Giant Rat prepare for company.
Gnolls will eat you. This is Plan A and they don't have much of a backup. Once they get your scent they'll gang up on the weakest in the herd and try to pin him down so they can start dinner. If the rest of the party puts up a fight they'll make sure their meal gets dragged with them while they retreat.
When your players tire of dying and running away perhaps they'll think of a better way to get the treasure.