Anyone familiar with American banking and instant-access savings?

I maintain a bank account in the US because I occasionally receive payments by cheque in dollars and occasionally need to write cheques in dollars. Trouble is, my bank account tends to accumulate more than
I spend - and the account, doesn't pay me interest.
The bank I use does have a basic savings account but it requires a minimum balance in order to avoid charges. Can anyone suggest where I can switch my dollar balane to during times when I'm not doing any spending? I need to be able to get instant access to my money and I don;t want to have to pay charges when I'm not using the "growth account" whatever form that might take.
I also have an account with Ameritrade which I haven't used for several years. Is there some low-risk definite-growth investment I could access via that. If so, I could use Ameritrade instead of my US bank, since they do provide a chequebook, actually.
Many thanks,
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