Balloon

A balloon was an inflatable bag filled with gas. It had a wide variety of purposes.

Transportation
In the 17th century, the Eleventh Doctor escaped from the Tower of London in a hot-air balloon, which witnesses called a "magical sphere". (DW: The Impossible Astronaut)

In 1851, Jackson Lake used a hot-air balloon called a TARDIS. (DW: The Next Doctor)

In 1863, the Third Doctor and Charlie Fisher used a hot-air balloon to help sketch a map of the Confederate lines. (TVA: Backtime)

In the 1940s, barrage balloons were used in the United Kingdom during World War II. (DW: The Empty Child)

In the 21st century, a picture of a hot-air balloon was on display in a space museum. (DW: The Seeds of Death)

On Spiridon, the Third Doctor improvised a hot-air balloon from plastic sheeting, enabling him and the Thals to escape the Daleks. (DW: Planet of the Daleks)

When the Sixth Doctor took the TARDIS into Astrolabus' realm, it passed by a hot-air balloon. (DWM: Once Upon a Time Lord)

Science
In the 1970s, Dr. Tyler used balloons to carry devices which monitored cosmic rays. One of these accidentally picked up a gell guard. (DW: The Three Doctors)

While on Storm Mine 4, Chub carried out experiments with weather balloons. (DW: The Robots of Death)

Toys
In 1913, Daughter of Mine carried a red balloon with her. (DW: Human Nature)

In 1914, Aphasia carried a red balloon with her. (NA: Human Nature)

As a child, Alistair Gordon Lethbridge-Stewart lost a red balloon. The Eighth Doctor later changed time to a minor extent so that he caught the balloon and returned it. (EDA: The Shadows of Avalon)

Balloons were often used by clowns, such as Odd Bob the Clown, (SJA: The Day of the Clown) Clara the Clown, (DW: The Celestial Toymaker) and Chuckles and Boris. (K9TV: Robot Gladiators)