But sometimes what the players THINK is the most fun isn't. The thing is, sometimes it's fun to use a fireball to melt your way through ice. Sometimes it's more satisfying to stand in front of the door, have your wizard make an Arcana check to point the trap at the door, attract the attention of a trap with an Acrobatics check and succeed on your Athletics roll to jump away at the last second in order melt the ice.
What players might get disappointed by("What do you mean my fireball can't melt the ice?") ends up being MORE fun for them in the end. Mostly because they had a chance to fail and succeeded anyway. This is especially true when a solution obviously favors one class or player over another. It sucks to be the fighter when you realize that a Wizard can simply say "I use a fireball, that solves everything and we go". Some people see that as creative. I see it as simply looking at your character sheet and picking a power.
Maybe yes, maybe no. A fireball doesn't solve 'everything', but does seem a pretty obvious solution for a wall of ice.
I'm not going to disagree entirely, as there is honestly some truth to what you say - I talked before about how I often use keywords on powers as guidance for what creative stunts those powers might be used for. And I did quickly realize that arcane and divine powers typically come loaded with a lot more keywords than martial powers - should that mean the casters can simply do more stuff? I've worried about it less since seeing that martial characters tend to be more capable of physical stunts using acrobatics and athletics, but I can still see the fear that too much leeway falls back into 'casters can overcome everything, and no one else needs to show up.'
But in the end, stunts and creative use of skills and powers is subject to enough DM ruling that you can usually keep things in balance.
To address your original question - is it more fun to fireball the wall of ice, or get to try out the trick of setting off the trap and then dodging out of the way as it burns through the wall of ice? I honestly don't know. I do know that situations with
only one solution are a bad thing, and
here is a very good article on why.
For any given obstacle, it is perfectly fine to plot out what the best option might be for the PCs to overcome the obstacle. Using a trap to blast through a frozen door, or joining the city watch to be allowed into the quarantined part of the city, or finding the secret password to get past the guards. Go ahead and set these up as the easiest solutions, or ones that provide extra benefits if the PCs go through with them. Feel free to emphasize them and steer the PCs towards them. Make other options really challenging to pull off, sure. But the second you decide there is only one option, and everything else fails... you've made a mistake.
Because it will be frustrating when the PCs try to do something else, that should work, and you shut it down. Because they aren't getting to play the game anymore, but simply following a carefully prepared script for you - which you might be really excited about, and think will make for great scenes that everyone will enjoy! And maybe it will - maybe you can even get away most of the time with convincing the party that they had choice in the matter. But I've seen a lot of games where players get frustrated as they do discover their lack of agency.
"But sometimes what the players THINK is the most fun isn't."
This could well be true. But I really recommend against deciding
for the players what will be the most fun, and forcing it upon them.