Disney Infinity

You do not need to do the character-specific challenges for this, only the normal ones, so you don't need to buy any additional characters to get this.

The Pirates set has only 8 challenges that aren't tied to a character. Some are collectible, some involve killing enemies, one involves using only bombs and one involves your cannon dinghy. (I won't lie, that thing's awesome.) There are no bike or driving missions, which worked better for me since you can't maneuver them as well as a character. There are also no races.

It's going to depend on your style of play, of course, but I just found Pirates the easiest.

Edit: I noted this elsewhere, but I should include it here. The last mission that I found, Shootin' Up the Swamp, didn't appear until after I did a few missions with Tia Dalma after the main story was finished. KeyWestSniper says specifically that it unlocks after you gold star "Sink the Scurvy Rats". I don't know if you have to keep going for any of the other challenges as well, but if you can't find one, that may be why.

Here's a list of the 8 challenges (thanks Zulghinlour for the specifics on each:

Rally on the Fort - Found in Fort St. Grande, halfway up the fort - Collector (30/50/70)

Scurry on the Shipwreck - Found in Shipwreck Shoals (seen from the dock) - Collector (25/50/70)

Pitching with Power - Found in Dead Man's Cove - Collector but must be destroyed with bombs (10/15/20) time limit 2:30

Clear Out Dead Man's Cove - Found in Dead Man's Cove (seen from the dock) - Combat (3/6/10) time limit 3:00

Buccaneer Bay Ballyhoo - Found in Buccaneer Bay - Collector (30/45/60) time limit 1:30

Buccaneer Break - Found in Buccaneer Bay - Collector (20/35/50) time limit 1:30

Pantano Bayou Dash - Found in Pantano Bayou in the far back section via dinghy - Collector (40/50/60)

Shootin' Up the Swamp - Found in Pantano Bayou in the far back section via dinghy - Collector but must be destroyed with dinghy (10/15/21) time limit 2:00

You don't have to do these in any particular order.

Ok, so the playset that I picked for this one is The Incredibles and here's why:

-It's an open world-style playset that is one single, easy to navigate map
-The enemies are far less annoying than Monsters University
-It's the first playset that I played and it's super-fun

It turns out that Pirates has fewer challenges to complete than Incredibles, but I didn't realize that until after I completed all of The Incredibles challenges on hard. Before attempting to complete these on hard, make sure you have completed all story missions, unlocked all of the gadgets and vehicles. All of these challenges are attainable with Mr. Incredible who is in the starter pack. Challenges for the other Incredibles characters do not count toward this achievement.

For the collectible challenges just try and figure out where the path you're supposed to take is. The platforming controls aren't that great- just make sure to not aim LS when jumping straight up from one ledge to another.

For driving challenges (by far the worst part of this) do doughnuts before you start the challenge to fill your boost meter and look for a shortcut whenever possible (you don't have to stay on the road)

For combat challenges, make sure you're using Mr. Incredible and throw enemies off of the rooftops whenever possible. Before doing the harder combat challenges, try flying around in the helicopter (once unlocked) and look for the metal boxes on the rooftops. When destroyed, they will occasionally release a superhero that will fly to your headquarters. Once there, you can walk up to them and press X to activate their help for a short time whenever you battle (they will fly in and beat up your enemies during combat, the more the merrier).

For the gliding challenges, just hold Y to glide and LT to dive. Dive and release. Once at the top of your arc, dive again. Rinse and repeat.

For hoverboard challenges, just remember that after you float off of a ledge, you have three seconds before your board will drop and you can kickflip to extend the time on the horizontal plane.

For helicopter challenges, just be patient and take it slow... you are given plenty of time to get these done. On a personal note, the helicopter controls in this game make me want to kill myself. I got used to them and I'm dreading piloting a chopper in BF down the road.

It can be annoying to locate all of the challenges that you need to do, just keep traveling across the city looking for the yellow light from the challenge circles.
I wish you all luck, please comment if you have any suggestions for the solution and I'll fix it if necessary.

I did this one playing The Lone Ranger Play Set. There are 4 simple horse race challenges, 2 character specific challenges, and then 4 challenges requiring the Crow Wing Pack which lets you fly. You have to hunt down 5 crow towers to unlock the pack, but they're easy to find, it's a small sandbox.

From what I've read, you don't need to do the character specific challenges, but I did them anyway to help level up my characters.

The horse races are straightforward, the hard levels on each require you to find shortcuts to lower your time, but again, very straightforward.

The Crow Wing Pack challenges require you to fly as a crow and collect floating collectibles. The controls are a bit awkward at first but the good news is your hitbox is large, so you don't have to fly directly into each collectible, if you touch it with the edge of your wings it still counts. At the hard difficulty you'll need to collect EVERY collectible in the challenge. These shouldn't take you long to complete.

In order to access all of these, you'll need to complete enough story missions to reach the third area, the railroad camp. The whole Play Set doesn't take long to complete.