I didn't have anything to do today so I had a lot of free time.
I didn't have much work to do today.
I was at work today but I didn't have much to do.
「I had not work」は少しあいまいですね。「会社が休みだった」という意味にもなりますのでもう少し詳しく説明した方が良いと思います。例えば、
「やることがなかった」＝ I didn't have much to do/I didn't have anything to do/I didn't have any work to do
「暇だった」＝ I had a lot of free time
私の意見では、こちらの文章で説明した方がいいと思います：
I was at work today but I didn't have much to do.
その場合の「仕事がない」はこのようにもいいます：
(1) I was free at work today.
「今日は会社で暇でした」
(2) I had a lot of free time at work today.
「今日は会社で暇がたくさんありました」
どちらの文も「暇」のfree（形）またはfree time（名詞）の言葉があります。
at workは「職場で」という意味なので、「会社で」の意味にもなります。
1. We use this phrase to explain the work was not 'heavy.'
Light work = easy work or not very much work; only lightweight work (if the materials are not physically heavy).
2. Not much to do = very little work to do.
"There wasn't much to do at work so we played cards until we went home."
1. これは、仕事がそこまできつくなかった、ことを表すときに使います。
Light work = 簡単な仕事、そこまできつくない仕事、軽い仕事
2. Not much to do = very little work to do. することがあまりない
"There wasn't much to do at work so we played cards until we went home."
職場でやることがなかったので、帰るまでトランプしてた。
e.g.conversation
person 1....I was going to call you but I thought you'd be working.
person 2....I was free all day, I didn't have any work to do, you should have called.
例）
conversation
会話で。
person 1....I was going to call you but I thought you'd be working.
人１：電話しようとしてたんだけど、仕事してると思って！
person 2....I was free all day, I didn't have any work to do, you should have called.
人２：１日中暇だったよ、何もする仕事がなかったんだ、あなたは電話するべきだったね。
Nothing special. (most common) You went into work today but it was just a normal day where you turn
up, do your job, and go home. Great expression as it will move the conversation on to another subject. " how was your day at work?" "It was ok, nothing special though".
Standard - you went to work, did your job, went home. Nothing really happened.
Not bad - normally you can answer by saying this, and then ask "how was your day?".
Nothing special
-（最も一般的な）あなたは今日仕事に行きましたが、仕事をして家に帰る。 それは会話を別の項目に移すときのように素晴らしい表現です。
"how was your work"
-仕事はどうでしたか"、"It was ok,nothing special though"それは大丈夫でしたが、特別なことはありませんでした"。
Standard
- あなたは仕事に行きました、家に帰りました。
本当に何も起こりませんでした。
Not bad- 通常、これを答えると、「how was your day」と聞くことができます。
確かにI had no workだと休みだと勘違いされてしまうかもしれませんのでI had no work to do todayの方がいいです。
It was a really slow day =すごい暇な日だった
仕事がない訳ではなく、暇な日だったと言う意味です。
There was hardly any work to do =あまり仕事がなかった
こちらも「仕事には行ったけど仕事があまりなかった」と言う意味です。
To explain your situation, you should use one of these two sentences. Both of them are talking about the same subject but from a different viewpoint. I would choose the first one.
イギリス 
アメリカ合衆国 
日本 
