I'll take stupid Terraform tricks for $500, Alex

01-main.tf

module "my_instance" {
  source = "../path/to/crazy_module.tf"
  ...
  count = 1
  include_eip = "true"
}

02-crazy_module.tf

resource "aws_instance" "instance" {
  ...
  count = "${var.count}"
} 

resource "aws_eip" "instance" {
  depends_on = [ "aws_security_group.instance" ]
  count      = "${replace(replace(var.include_eip, "/\\Atrue\\z/", "1"), "/\\A[^1].*\\z/", "0") * var.count}"
}

resource "aws_eip_association" "instance" {
  instance_id   = "${element(aws_instance.instance.*.id, count.index)}"
  allocation_id = "${element(aws_eip.instance.*.id, count.index)}"
  count         = "${replace(replace(var.include_eip, "/\\Atrue\\z/", "1"), "/\\A[^1].*\\z/", "0") * var.count}"
  depends_on    = ["aws_eip.instance"]
}

my variation:

variable "truefalse_map" {
  type = "map"
  default = {
    "1"     = ""
    "0"     = 0
    "true"  = ""
    "false" = 0
  }
}
...
# in a resource ...
...
  count = "${coalesce(lookup(var.truefalse_map, var.attach_ebs), var.quantity)}"
...

Then I'm using 'attach_ebs' as a flag, haha

@jc-asdf: Hah! I almost did something like that too. The reason I didn't was that I didn't want to have to define that truefalse map in every module that used my hacky conditional logic, even though I think your approach is a little more readable. :)