I'm a new DM working with a group of brand-new players. It's going very well so far. The campaign is pretty railroady to be honest but I don't think they've noticed (any decision they make just tends to point towards the same result, but I think I've kept it subtle enough.) and they're enjoying it regardless.

I wanted to run an idea I had for an adventure I've come up with by you. This is supposed to be something fun and lighthearted as a break after a spooky horror ghosty haunty thing. Keep in mind these are brand new players who have never done tabletop RPG or even really been into fantasy anything before but are absolutely loving it.

A green dragon has put a geas on the party in exchange for letting them explore a dungeon it controls to look for the macguffin. After exploring the dungeon they must go to the green dragon's rival's lair and steal a powerful object for the green dragon.

The rival is a copper dragon and his lair turns out to be a beautiful magnificent mansion that acts as a 'hotel' for creatures from lots of different locations and planes. An old man greets them and tells them the master will be out for awhile but feel free to make themselves at home as long as they don't disturb the other guests. In conversation the old man also drops hints that the master keeps it's hoard in the north tower (or wherever).

Upon infiltrating the north tower, the group is met with a number of rooms full of puzzles, riddles and challenges they must pass before coming to the final room where they find the powerful object along with a number of other powerful magic items.

Now here's the part that might be entertaining or might be annoying: All the objects are mimics. After a round or two of hopeless combat the old man appears and transforms: Turns out he's been the copper dragon the whole time! He gets rid of the mimics, congratulates the party for being so entertaining and making it t the last room and rewards them with a lesser magic item. Upon finding out why they were trying to steal from his lair he laughs at the foolishness of letting a green dragon put a geas on you and uses his far more formidable power (The green dragon is adult, the copper is ancient) to break the geas.

Your opinions? If you feel that it would be annoying can you suggest a better way to handle it? Knowing the group I feel like they would enjoy it but if it only sounds entertaining to me and you all think it's annoying then I'll concede my opinion is likely wrong and I should change it.