To say "safe travels" in Japanese, you can use the phrase "良い旅を" (yoi tabi wo). This expression conveys a wish for someone to have a good or pleasant journey.
If you want to emphasize safety more specifically, you might also say "安全な旅を" (anzen na tabi wo), which translates to "a safe journey." Both phrases are appropriate depending on the context of your conversation.
When offering this sentiment, it's common to speak in a warm and friendly tone, especially if you're addressing someone close to you. Japanese culture places a strong emphasis on expressing good wishes and care for others, so using these phrases will surely be appreciated.