Das Schlimmste war das Besuchen von dem Geschichtemuseum. (the gerund [visiting] is difficult in German - I'm not certain if , zu besuchen works in the past like that, the way I have written should.)
I'd say to stick with 'das Besuchen' as well. It does seem to sort of work, but you'd probably be safer using what Inexorably suggested. Ich fand das sehr.. not Ich finde (past tense)
liebte alle von den Artefakten (all of artifacts doesn't make sense)
also ging Leah und ich... (verb has to go straight after also)
aber es war nicht alles schön - what do you mean here?
You put in the English translation that 'there wasn't anything nice', so you could say 'aber es gab nichts Besonderes' Dann wir in das Cafe gingen hatte ich eine Cola > what are you trying to say here? "Then we went to the cafe and i had a cola"? If so, think about where the verb gingen would go and then add the und and think about where hatte would go. Cola is also neutral
Auf der Reise zurück zu Leah's Hauses > I'm not certain if this how you say 'on the trip back to', but you don't use the apostrophe in German like this. Leahs Haus (why Hauses?) is correct.
I think that you use nach instead of zu brach das Auto - if you want to speak about a car breaking down say, das Auto hat(te in past) eine Panne.
so dass wir spät nach Hause kam - what do you mean here?
Use 'also' here, so 'also kam wir spät nach Hause'. warden = werden
Sie werden für einen Monat in London bleiben.
'Leah ist freue mit darauf' doesn't make sense - do you mean 'Leah freut sich darauf'? werd = wird
I think it's 'they will' here, so 'werden' die konnte = sie konnte
"nach Brighton...etc." sounds wrong, what do you mean by nach?
Pretty sure you don't use 'nach' with 'besuchen' spaß sind = Spaß machen
There's also a comma before 'weil' verbessern has an r
neue Freunde (plural ending) machen
Leahs Schule
After ‘Sie hat vier Lektionen, die’, send the verb to the end (I think that you meant to say ‘dauert’ instead of ‘zuletzt’).would remove es from ''denn sie liebt es''
Lieblingsfach should go to the plural as you're mentioning two subjects.
Capitalise all nouns and uncapitalise verbs.
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I'm uncertain about a few things (whether they're correct/incorrect) so
@TheOtherSide pls send any help you can =] especially about whether the , zu besuchen works in past tense?
wrong person, i mean
@TheOtherSide.