Bicycle bubbles with a clear sound and easy to install. The Trivio and the Niet Verkeerd Bell are simple bells with a reasonable sound.
The bubbles from Guee have a handy turning system so that you can turn them on your trade or on your gear wire.
Do you have a Wahoo and do you want to integrate everything? Then you can go for the HideMyBell variant where the bell is hidden under the holder of the Wahoo Navigation system.