Type of account for investing surplus funds when planning to retiring early?

You mention IRAs and 401(k)s so I'm assuming you're based in the US. There is no requirement that you wait for retirement age to begin drawing on your retirement accounts-- you can happily start drawing on it early via substantially equal periodic payments. That's going to be based on your age and the size of the account but you're getting basically the same amount every year which is roughly what you'd want to happen in retirement.

Beyond that, though, most people that retire early are going to have a fairly large amount invested in a regular old brokerage/ mutual fund account. It's not tax advantaged. But it is very flexible. You can withdraw whatever you want whenever you want as long as you pay your taxes. Ideally, mostly long-term capital gains taxes.

Many people who plan to retire early have, or put, substantial assets into non-tax-advantaged accounts (regular brokerage and savings accounts, real estate, etc.).

Some people, including me, plan to use a Roth conversion ladder. You can search for the details, but in brief:

1. Start: with 5 years of accessible money.
2. In years 1-5: Convert 1 years of expenses from a traditional to a Roth IRA. 3, In years 6+: Convert 1 years of expenses from a traditional to a Roth IRA. Withdraw the aged, converted contribution from 5 years earlier.