Nice studio flat, ideal situation, and nice host.

For (the) St Valentin’s day we planned to go to Nice’s carnival but because of weather(s) conditions and rain(ing), the carnival’s processions were canceled. Even so we had a good time, and (the) Did’s apartment made up for it. (The St Valentin’s day went off pleasantly partly because) It was a nice and convenient place and as a result, we had a lovely St Valentin's day.  For (the) lunch, we bought a chocolate mousse cake in the pastry shop, Serein, opposite to the Monopry, and it was absolutely delicious. In addition, Did (indicated) showed us a cheap (butchery) butchers shop with tasty meat near the flat.

The flat (has an) is ideal (situation) because it is near everything in (the) old Nice, and (the many) even though there are a lot of stairs (to reach) up to the flat, they are quickly forgotten and actually help to keep us (in form) fit/in shape! There are a lot of restaurants, bars, and shops that we can reach in less than 5 minutes, walking, and of course (including) the pretty streets of (the) old Nice (which are everywhere,) and the sea are also very nearby. Despite the (perfect situation) central location, the flat was very quiet! (And) It also offered us a beautiful view over the rooftops.

Did (got) gave us a (pleasant) warm welcome, taking the time to give us recommendations, (presenting us his apartment and himself.) introducing himself and showing us around the apartment. He offered us a bottle of wine and he left (available) a basket with various kinds of tea, each one more appetizing than (others) the last, and fruit(s). In addition, he was very flexible (on) with his schedule. Thanks a lot, Did.

This is very well written and shows that you have a very good grasp of the language. Most of the corrections I've made are more about making the English more natural, rather than grammer issues. For example, "pleasant" isn't used very much by many English speakers unless describing a person, so in your Notebook Entry I've replaced it with "lovely" which is used far more often, and a "warm welcome" rather than a "pleasant welcome", as a "warm welcome" is a very common English phrase of speech. Also, watch out for putting "the" where it isn't needed, this was your most common mistake.

It sounds like you had a lovely time, and this is a great entry, well done! Feel free to message me if you have any other questions :)