Which international roaming add-ons are known for simple activation and pay-as-you-go options?

Some international roaming add-ons that are commonly known for simple activation and pay-as-you-go / usage-based options include:

• Google Fi — very easy to activate, with international roaming included in many plans and straightforward usage-based billing in supported countries.
• T-Mobile International Roaming — generally simple to turn on; many plans include basic international roaming, with optional data passes and pay-per-use options depending on destination.
• Vodafone Roam Further / Roam Abroad — often easy to add through the app or account portal, with day-pass and pay-per-use style options in some regions.
• AT&T International Day Pass — simple activation, especially for short trips; it’s more of a daily pass than pure pay-as-you-go, but easy to use.
• Verizon TravelPass — similarly easy to activate and use, billed by day in covered countries.
• AIS / DTAC / TrueMove (travel SIM or roaming packs) — in many Asian markets, these providers offer very easy app-based roaming add-ons with prepaid, usage-based bundles.

If you want, I can also list the best options by country/carrier or separate them into:

1. true pay-as-you-go,
2. day-pass style, and
3. prepaid roaming bundles.