Kami's place is very up in the air, there is no way to come by walking.
Kami's lookout is been like meeting place also most time Shenron is called by there.
thath place is later Dende's place when Piccolo and Kami do fusion which made Piccolo much stronger than before.
Master Roshi's Island
Master Roshi lives here, this is very "famous" place where Z-Warriors usually gather.
Z-Warriors train Roshi's place some times, also Krillin lived there with #18.
King Kai's Planet
First Z-Warrior who arrived King Kai's planet was Goku.
King Kai thinked thath goku may learn many great moves what even King Kai cant learn.
He teached to goku kaioken low level to use because time was running out. Also Goku learned here the famous spirit bomb.
After Nappa and Vegeta arrived to planet earth and exiled, Piccolo, Tien, Yamcha, Chaozu moved to King Kai's planet to train also.
King Kai's beautiful planet get destroyed when Goku teleport to King Kai's planet with Cell at once Cell is doing
This is Supreme Kais planet very peaceful and beatyful planet where is seldom fights.
when Supreme Kai and Kibito leaved to Kaioshins planet on majin buu saga, they taked Gohan with them,
and trained him. Also Gohan pulled Z-Sword out even Supreme Kai wasn't able to pull Z-Sword out.
Room of Spirit and Time/Hyperbolic Time Chamber
This is very great place to train. One year in the time chamber is just one day on real world!
You can go to time chamber two times, if you go more than thath you will be trapped forever to chamber.
Also, if you destroy the entrance door, it won't be very nice also if you destroy the sandglass of thath chamber :).