What pleasant surprises did David get one afternoon?

It is indeed a most pleasant surprise for David when Mr. Pegotty and Ham unexpectedly show up. Their arrival is a reminder of what's most important to young master Copperfield, and presents a refreshing contrast to the myriad privations and indignities of school life.
The unpleasant episode with Mr. Mell reveals Steerforth to be an arrogant, entitled spoiled brat, determined to get his own way even if it means getting someone fired. Though he's still somewhat dazzled by Steerforth, David's been given a frightening glimpse into his friend's true nature. He's been given a warning that he would do well to heed. Steerforth is just a fair-weather friend, a user and a manipulator. Mr. Peggoty and Ham, on the other hand, are real friends to David. They are good, simple people, kind-hearted souls who'll gladly help anyone in need.
David's still too naive in the ways of the world to understand it just yet, but Steerforth's gesture of generosity to Mrs. Mell is completely phony; it's all about showing off how much money he has and exercising control over those he regards as his social inferiors. By contrast, the gift of food that Mr. Peggotty and Ham bring David is a genuine expression of their warmth and friendship. Unlike Steerforth, there is no affectation with them; what you see is what you get.

The pleasant surprise David receives can be found in Chapter 7.
After an especially tense exchange between Mr. Mell, Steerfoth, and Mr. Creakle, David is despondent. For his part, Steerforth has managed to manipulate Mr. Creakle into firing Mr. Mell.
Despite his initial reservations about Steerforth's behavior, David can't help but admire his mentor's promise to send Mr. Mell's mother some money.
After this sad interlude in David's life, he gets a pleasant surprise one afternoon. David overhears Tungay telling Mr. Creakle that he has some visitors. Upon entering the parlor, he discovers that his surprise visitors are none other than Mr. Peggotty and Ham.
The two men bring news about Peggotty and Emily to David, of which he is very happy to receive. They also bring gifts to him: two huge lobsters, an enormous crab, and a large bag of shrimps, all boiled. Later, David shares his bounty with Traddles and Steerforth.
So, the pleasant surprise David receives is a visit from Mr. Peggotty and Ham.