Are there any Virtuous deeds. How long do Yakshas live in comparison to normal human beings. What powers do they possess?
Yakṣa (यक्ष) according to the 2nd century Mahāprajñāpāramitāśāstra (chapter XX). Accordingly, “he knows that a jealous man who loves to dispute but who can give fine houses (gṛha), beds and seats (śayāsana), clothing (vastra) and food (āhāra), will be reborn among the Yakṣas who fly about in palaces and temples where they enjoy all kinds of pleasures and material advantages”.
As per puranic Encyclodpedia.
Yakshas are a class of Upadevas (Semi-Gods).
There are ten types of Upadevas, namely - Vidyadhara, Apsara, Yaksha, Rakshasa, Gandharva, Kinnara, Pisaca, Guhyaka, Siddha and Bhuta.
Yakshas and Rakshasas are related. Yakshas are the progeny of the Sage Pulastya.
O best of Kings, the offsprings of the Pulastya of great wisdom were the Rakshasas, Monkeys, Kinnaras (half-men and half-horses) and Yakshas. - Mahabharata, Adi Parva, Chapter 66, Verse 7
Kuvera is the king of Yakshas. Many Yakshas remain in the assembly of the Kuvera (the lord of treasures). They worship him.
There also came the lord Kuvera, having a body resembling pure gold, seated on a highly splendid car and accompanied by a numberless Yakshas. - Mahabharata Vana Parva, Chapter 41, Verse 7.
The Gandharvas called Kinnaras, and others called Naras, and Manibhadra, and Dhanada, and Swetabhadra and Guhyaka; Kaseraka, Gandakandu, and the mighty Pradyota; Kustumvuru, Pisacha, Gajakarna, and Visalaka, Varaha-Karna, Tamraushtica, Falkaksha, and Falodaka; Hansachuda, Sikhavarta, Vibhishana, Pushpanana, Pingalaka, Sonitoda and Pravalaka; Vrikshavaspa-niketa, and Chiravasas these O Bharata, and many other Yakshas by hundred and thousands (always wait upon Kuvera). - Mahabharata Sava Parva, Chapter 10, Verse 14-18.
You can read more about Yaksas from the Puranic Encyclopedia by Vettam Mani.