a) Have you seen Temmie Village? If you missed out the first round, try finding it this round. =p (hint: it's accessed at the glowing mushroom place)

b) You can access Sans' room and hideout by resetting 2~3 times after hearing his Judgment speech. Sans' speech will change, it won't be hard to miss!

c) Remember the floating flower puzzle? Laying the flowers on the bottom right will lead you to a signboard, right? But there's a secret place you can access at bottom left if you lay the flowers correctly. There is a goodie there~ xD

d) Crap, you'd miss this... but if you call Toriel when the Annoying Dog is in your inventory, you'll... um... discover the reason why Toriel hasn't been answering your calls.

e) If you backtrack, sometimes you'll see Flowey slipping away... I'm not sure if it's available in 2nd playthrough tho, considering you and Flowey are in okay terms, but on that particular corridor with a lot of Echo Flowers... one of the flowers will have its dialogue changed.

f) If you buy hotdogs from sans and your inventory is full. He will put it on your head. This can go on to 30 times.

who is also Mettaton now... and Napsta is Metta's biggest fan! D;