我的妹妹叫玛丽。她从两年前开始学习法语，终于今天下午她去欧洲旅游了。玛丽决定了把不太大红色的行李箱装好，因为她一共只能住在法国五天一共。她打算游览很多很棒的名胜古迹。在巴黎食物虽然比较贵的，但是玛丽太喜欢法国菜了，所以她首先到了餐馆尝尝具有民族风味的菜。我希望她当回家时是非常满意的，并且顺便给我带一个漂亮的纪念品。


Ok, first of all, if you want to say she has studied French since two years ago,  you should say "她是两年前开始学法语的" or "她从两年前开始学法语" but not "她是两年前开始学法语"
And then the expression of  "越多越好的名胜古迹" seems no sense to me. Do you want to say "many and nice places of historic interests? " if that so, you should say "很多很美的名胜古迹or很多很棒的名胜古迹".  Because "越多越好" means "the more,the better ."
 'Mary likes French food very much, so she go to the restaurant to taste the local food.' In my view, in this sentenceis is completed, so if you add "首先" in the sentence, which means 'first' in chinese, you better add more contents in it, like what then etc. Beside, the word "访问" is too official to use in this sentence, "到了" is good.
"顺手" is the word which more used in the situation when something near by you and need you to get it or take it. 
But "顺便" is more correctly when you want someone do you a little favor.

我有一个妹妹叫玛丽。她是两年前开始学习法语的，今天下午她终于去欧洲旅游了。玛丽决定把不太像是大红色的行李箱装好，因为她只能住在法国一共五天。她打算游览许多的名胜古迹。巴黎的食物虽然比较贵，但是玛丽太喜欢法国菜。 所以她首先访问餐馆，尝尝民族风味菜。我希望她回家会感到非常满意，并随手给我带一个漂亮的纪念品。