Aujourd’hui j’ai rencontré mon amie Susan pour déjeuner et assister à un atelier d’art.

I think in french we actually make a difference whether you attend to a theoretical class ("assister") or to a practical one ("participer").
I guess you might use both verbs in any circumstances, but the first one gives a little passive taste to the experience (seated and listening) while the latter is more active (hands in the clay or paint).

Nous avons choisi un restaurant italien près du centre communautaire où se déroule l’atelier.

I guess you wrote this text after the workshop.
If the workshop is still issued there, present tense is fine (the restaurant still exists and is still near the workshop's place), otherwise you'll have to go with past tense ("the restaurant was near the workshop", "the restaurant is near where the workshop was" : i both cases it will be "où se déroulait" ).

Après que nous sommeyons arrivées et que nous nous sommeyons assises, Susan a dit, « Je suis flexible.

"Après nous" : it took me some time to understand this was a straight translation of "After we something".
In French "après <event>" means "after the event" ("Après la pluie.", "Après le déluge.", or when the event is an action, "Après que nous aurons bu."(future) "Après avoir bu."(past).)
If there is no provided event (just like in your sentence), the event is implicitly that of the former sentence (exemple : "J'ai bu une limonade. Après j'avais mal au ventre." after drinking the lemonade I had a soar stomach.). In the context, I was a bit lost finding the implicit event :)
"Après que nous soyons arrivées" : the event is our arrival.

The tenses in that sentence might be a bit hardcore for the moment though, and I have to confess they would also feel a bit literary for french people too. So I suggest a workaround : "Une fois arrivées et assises, ".

"Flexible" is ok-ish, but we would rather use this word in a physical meaning (rubber is « flexible »). In the context we would rather say "je suis ouverte" (beware the double meaning for dirty minds), "je suis ouverte à toute suggestion" (that would cut short on every dirty mind), "je suis « open »", or simply "je te fais confiance" or "je te laisse choisir.".

Tu commandes ».

Might be a bit ambiguous, the first meaning of "Commander" would be giving orders to a subordinate (to be in charge).
"Je te laisses choisir.", "Je te laisse passer commande."

Il y avait des commentaires positifs sur lea pizza aux aubergines et les rigatonis.

My spellchecker wants to remove the "s" from "rigatonis", I guess he is a bit snob and would argue that the "i" of rigatoni already shows a plural, the same way we shouldn't say "des scenario" but "des scenari"... I would object that once you use words from a foreign language in your own language, they cease to be part of their original language :)
Long story short : let's ignore the snobish spellcheckers, keep "s" at "rigatonis" and write "scenarios" \o/ :)

Nous profitonsavons apprécié le repas, mais nous avons trouvé les rigatonis trop épicés.

- Why use present and past in the same sentence ?
- In french we would rather say "apprécier" (enjoyed) than "profité" (we benefited from, we took advantage of).
- "mais" is OK but maybe a bit too factual here. Maybe you want to use "même si nous avons" (even though) "bien que nous ayons" (despite the fact that we)

À douze heures quarante -cinq nous sommes parties pour l’atelier d’art.

Avant que le cours ane commencée, j’ai montré aux élèves assis à ma table le travail que j’avais fait chez moi.

alternative : "Avant le début du cour"

Ils ont tréété très impressionné.

«» Je suis une vrai frimeuse.