I was only marginally interested in what he showed me.
I wasn't that interested in what he showed me.
marginal interest だとちょっと不自然になってしまいますが、marginally interested in ... だったら使えるかなと思います。
ーI was only marginally interested in what he showed me.
「彼が見せてくれたものにほんの少ししか興味がなかった」
ーI wasn't that interested in what he showed me.
「彼が見せてくれたものにそんなに興味がなかった」
I wasn't that interested in ... 「…にそんなに興味がなかった」を使って言った方が自然です。
ご参考まで！
カナダ 
