I am a Dungeon Master running a Dungeons and Dragons campaign. My players are all level 7 and I would like to reward them with magical items for completing a difficult puzzle. What are appropriate rewards for a Paladin, a Bard, a Cleric, a Warlock and a Rogue?
Paladin: Holy Avenger - A powerful sword that glows with a bright light when evil is near. It deals an extra 2d10 radiant damage to fiends and undead and grants the user resistance to necrotic damage. It also has the ability to cast the detect evil and good spell once per day.

Bard: Instrument of the Bards - This magical instrument comes in several varieties (such as the Doss Lute, Fochlucan Bandore, or Mac-Fuirmidh Cittern) and grants the user a +2 bonus to performance checks. It also has the ability to cast a few spells, such as command or faerie fire, once per day.

Cleric: Staff of Healing - This staff allows the user to cast the cure wounds spell at 3rd level once per day without expending a spell slot. It also has 10 charges that can be used to cast lesser restoration (2 charges), greater restoration (5 charges), or mass cure wounds (5 charges).

Warlock: Rod of the Pact Keeper - This rod grants a +2 bonus to spell attack rolls and adds 2 to the DC of any saving throws against the warlock's spells. It also has the ability to regain a single expended spell slot of 5th level or lower once per day.

Rogue: Boots of Elvenkind - These boots grant advantage on Dexterity (Stealth) checks and reduce the sound of the user's footsteps, making it easier to move quietly. The user can also walk across difficult terrain without penalty and has advantage on any check to resist being knocked prone.
Can you also make up the puzzle for them to complete?
So, here's a puzzle that you could use for your players:

The Puzzle: The Chamber of Elemental Symbols

As the players approach the end of a long corridor, they come across a large, circular room with a raised platform in the center. The platform is surrounded by four smaller platforms, each with a different elemental symbol etched into the floor: a flame, a wave, a gust of wind, and a block of earth.

In the center of the room, on the raised platform, is a pedestal with a large crystal sitting atop it. The crystal glows with a faint, pulsing light.

The players soon notice that the room is divided into four sections, each corresponding to one of the elemental symbols. The sections are separated by invisible barriers that prevent the players from crossing into another section.

At the base of each of the elemental platforms, the players find a small box with a keyhole. The keys are scattered throughout the room and must be found in order to unlock the boxes.

Each box contains a puzzle that, once solved, will reveal a clue about the order in which the elemental symbols must be activated to unlock the crystal on the central pedestal.

The flame box puzzle involves rearranging a set of runes to create a phrase that means ignition.

The wave box puzzle requires the players to manipulate a series of mirrors to direct a beam of light onto a specific spot on the opposite wall.

The gust of wind box puzzle involves creating a gust of wind using a set of bellows to activate a series of wind chimes in a specific order.

The block of earth box puzzle requires the players to manipulate a set of stone blocks to create a specific pattern.

Once all four elemental boxes have been solved, the players will have a clue as to the order in which the elemental symbols must be activated. Activating the symbols in the correct order will cause the barriers to drop, allowing the players to access the central pedestal and claim their reward.