Time to go a-spelunkin’!

Yoshi won’t be scavenging for any gems and jewels in Mine-Cart Cave, but that’s okay: there’s plenty of Red Coins, Smiley Flowers, Poochy Pups, and Souvenirs instead! In case any elude your grasp, we at Hey Poor Player got the goods, so take a gander at the guide below!
Red Coins
As seen above, your first three Red Coins are located on the cliff reached by the waterfall. (3/20) Enter the cave and journey on until you reach another waterfall; ascend it, pop the flying cloud to your left, then flutter jump across the falls to earn another Red Coin. (4/20) After the Red Clock Cloud, descend the waterfalls and grab the three Red Coins to your left. (7/20) Continue on and ascend the rising falls to pop a flying cloud and grab four more coins. (11/20)
You’ll eventually find yourself in a waterfall deluged room; scan the background for a gold present, then smack it open for a Red Coin. (12/20) Afterwards, when you free the mine cart from its rock prison, hop aboard and jump above to discover a hidden cloud; pop it for another coin. (13/20) Once you’ve left the cart behind, travel on for some more waterfalls and shoot down the coin-carrying Fly Guy. (14/20) You’ll find another cart shortly after; hop aboard and unveil a coin-carrying cloud. (15/20)
When you’ve pushed the cart as far as it can go, platform over to the left until you uncover another hidden cloud for three more coins. (18/20) Finally, after you’ve conquered the massive waterfall, climb this totally-not suspicious cliff for a cloud hosting the two last coins. (20/20)
Smiley Flowers
Make your way through the level until you locate a Red Clock Cloud — ensure you have a healthy supply of eggs beforehand, then pop it and smack down the right-most crystal to fish out the Fly Guy holding your first Smiley Flower. Descend the following waterfall, then move on ahead until you run into some ascending falls; hop down for another Smiley Flower. Afterwards, bomb the rock wall above, then push the mine cart down to reach a hidden Smiley Flower below.
When you reach the waterfall-filled room, bomb the rock pillar with the Piranha Plant to uncover another flower. The next flower requires some back-and-forth action: free the mine cart with a bomb enemy, push it forward, then back until it hits a dead end. Swallow a bomb enemy, then hop into the hidden cave above and bomb more rocks to unearth a spring that conveniently places itself onto your cart. Push it forward again, then spring away into a hidden nook featuring a Smiley Flower! Exit, then reach the waterfalls and nab the Smiley Flower via an Egg Toss or a carefully-timed Flutter Jump.
Journey on until you open the door to reach a giant waterfall; pop the cloud open to reveal a cascade of coins, then ascend to reach a flower at the very top. Your last flower’s awaiting at the next mine cart — simply nab the floating trinket mid-ride, and you’re all done!
Poochy Pups
A trivial romp — you’ll find the first pup romping about some background falls (acquire it with an Egg Toss), the second imprisoned alongside a hidden mine cart (bomb them free!), and the third chilling with a Shy Guy atop another hidden mine cart. You’ve got this!
Souvenirs
Mine Cart with Flag: This abandoned mine cart may give you some trouble, as it’s hidden in the background — after the back-and-forth mine cart section, scan the background for its residence atop a tall rock. Look for a Flying Cloud followed by the Fly Guy Red Coin/Smiley Flower.
Crystal Clutters: The first one’s tucked away behind the first cart, the second by (coincidentally!) the second cart, and the third by (yet again!) the third cart’s railroad; the end of it, that is. You’ll find the fourth and fifth by giant waterfall, and the sixth by the final cart.
Bat Family: As seen above, you’ll find this flighty duo behind a Flipside waterfall sequence that’s not too far from the beginning. Their bright colors should give them away.
Caramel Billboard: In the distance by a bomb box. It’s in the area with the second mine cart.
Congrats, you’ve successfully plundered Mine-Cart Cave! Post in the comments below if you have any further questions, but in the meantime, check out our Yoshi’s Crafted World guide hub!