Sometimes は「いつもではない、または頻繁ではない」という意味です。

We usually played football at school, but sometimes we played hockey.

I sometimes wonder whether I should give up my job and just travel.

Sometime は、「未来または過去の未知または未定の時点」を意味します。

Why don’t you come over and have dinner with us sometime ? （今後の時期は未定）

I knew I had met her sometime , but I couldn’t remember when.