Nginx: directly return $remote_addr in text/plain

The simplest way is:

location /remote_addr {
    default_type text/plain;
    return 200 "$remote_addr\n";
}

No need to use any 3rd party module (echo, lua etc.)

Use ngx_echo:

location /ip {
    default_type  text/plain;
    echo $remote_addr;
}

Use ngx_lua:

location /b {
    default_type  text/plain;
    content_by_lua '
        ngx.say(ngx.var.remote_addr)
    ';
}

Here's an approach you can use to consider proxied connections:

# Map IP address to variable
map ":$http_x_forwarded_for" $IP_ADDR {
    ":" $remote_addr; # Forwarded for not set
    default $http_x_forwarded_for; # Forwarded for is set
}

server {
    ...

    location / {
        default_type text/plain;
        return 200 "$IP_ADDR";
    }
}