It is really nice to see that you want to write Finnish. I thought I'd help you a little as some of the forms are incorrect and nobody else has helped you out with this text yet. Since it has been 21 days, maybe you already know how to change the sentences to the correct form.

Aion mennä Särkänniemeen perjantaina tai lauantana, koska tyttäreni tykkää delfiineistä. The biggest changes for this sentence are the spelling of "Aion" because ajon means "of a drive" (it comes from the verb ajaa that has many meanings depending on the tcontext it has been used in), the spelling of Särkänniemeen because this form shows the intention of going there, the spelling of weekdays (in Finnish only names begin with capital letter), the spelling of "tyttäreni" because the words gains a double t (tt) when it is changed basic form of tytär and the spelling of delfiineistä (the word has double i (ii) and changes from the basic form delfiini into delfiineistä when it becomes "of the doplhins" instead of "dolphin").

Tallipihalla on myös tapahtumia. This one is the hard sentence because it is not connected to the first sentence so I am not totally sure what you wanted to say. This is my suggestion of what you could say but it still doesn't make it tie into the first sentence though. I started the sentence with the location because it makes more sense that way (as a native Finnish speaker I am ignorant of the grammar rule that explains why it makes more sense). I also changed the word because I have seen locals spell it like that in magazines (I think it is that ending because you are on the stable yard rather than in it). I also changed the word "tapahtuma" because this one is correct when you want to say that some place has events happening there at different times.

Sunnuntaina menemme kirkkoon ja sen jälkeen puistoon.So the changes were not big in this sentence and were about correct spelling. You did really well with this sentence.

Sitten lepäämme maanantaina. This one had just one spelling mistake. Really good again.

Haluaisin mennä äitini kanssa kylpylään ensi viikolla. Hän pitää uimisesta.

tai Haluaisin mennä äitini kanssa kylpylään ensi viikolla, koska hän pitää uimisesta.

These two sentences were mostly correct just the "she likes swimming" part needed a bit of changing. It was because hänen means hers so you didn't need to change the word at all. Pidä is the commanding form of the verb pitää which means to like or to hold (you could also use the word tykätä in the form tykkää). I wanted to show you two versions of the sentence because the second one is the more advanced version where you tie the two sentences together with the word koska=because).