Can I apply for a tourist visa after leaving Thailand on a volunteer visa?

Where are you staying in Thailand ?

As a citizen of Switzerland, you can leave Thailand and enter again on visa exempt, which will give you 45 days in Thailand without a visa.

If you're on the original 90 day visa you received from an embassy overseas, then you don't need to do anything except leave Thailand. If you're on an extension then you need to cancel your work permit and volunteer extension before you leave though or you could end up being fined 20,000 baht in the future for failing to cancel it since you signed a document that said you would do so.

The only problem you might run into is that volunteer visas and extensions are the most widely abused type and will likely cause extra scrutiny no matter what you do. But Laos is known as one of the easiest places to try and enter Thailand from, so it might be okay. You may still consider using a border run agency to take you across and bring you back though, to ensure a smooth process.

Depends on your visa extention record in thai.