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.