我去过威尼斯  “去了“ means 'went' or ‘have went,“去过” means 'have been', so they are different, u shall use "去了” here.

前天我和我男朋友一起去过威尼斯。same

威尼斯一直很好看，现在我去过六次威尼斯。It is a bit awkward...It might be better if change like 我一直觉得威尼斯很好看，现在我总共去过六次威尼斯。

早上我们坐公共汽车从Rimini到Chioggia，以后10点到威尼斯。“以后” has two meanings: 1. is almost the same as “然后”, but the position is different in a sentence. For instance, "坐车到XXX以后，再到XX。" and "坐车到XXX，然后到XX。" 2. nearly equal to "从此以后", which means from now on. eg. 以后都不理你啦！Therefore, u shall change "以后" to "然后”。

威尼斯的人不太浪漫,但是我们很高兴去了哪里！Conjunctions in Chinese are a bit different. Especially here, when u use "但是", dont forget "虽然". "虽然......但是....." they follow one another like shadow in a complete sentence. So, it is ”虽然威尼斯的人不太浪漫,但是我们很高兴去了那里！” “哪里” always refer to the place u dont know where it is, using in sentences of inquiry. Compared to "那里", it has a "口”， which means it has something to do with ur mouth (to ask). u might could memorise in this way.

Hope it is helpful, 加油！

Best,

Jewel