The work inside the orbiting laboratory came one day after astronauts Piers Sellers and Mike Fossum made repairs critical to the completion of the half-finished ISS.
NASA officials have been very pleased so far with the second flight since Columbia disintegrated over Texas in February 2003. Discovery suffered no damage during lift-off on July 4.
The astronauts emptied a container brought by Discovery last week filled with food, clothes, a laboratory freezer to store scientific samples, a large incubator to study plant growth in space and an oxygen generator system that will allow the International Space Station (ISS) to accommodate six crew members.
The Italian-made Multi-Purpose Logistics Module, dubbed "Leonardo", will now be filled with the ISS’s garbage and other discarded material that will be brought back to Earth on July 17.
The shuttle has been cleared of damage that would prevent it from coming home. However, an unprecedented final inspection will take place at the end of the 13-day mission to make sure Discovery was not hit by micrometeorites.
Sellers and Fossum prepared for their third excursion out of the ISS that will allow the astronauts to test material that would be used to fix the shuttle's heat shield if it had sustained damage.
Columbia's demise was caused by debris from its external fuel tank that pierced its heat shield during its launch, leading to the shuttle’s destruction as it returned to Earth.
The spacewalkers will use cracked samples of the shuttle's reinforced carbon-carbon (RCC) shield to test a sealant's effectiveness in orbit.
In their second spacewalk, the pair replaced broken hardware needed to build the station.
NASA officials said the repair set the stage for resuming construction of the station in the next shuttle mission. The US space agency plans to launch Atlantis late next month.
The Discovery mission is a final test of NASA's efforts to dramatically improve safety in order to resume regular shuttle launches and complete the ISS.