Whatever else one might say about Edward II, he was a generous uncle--at least to his favorite niece, Eleanor de Clare. Here's some of his recorded gifts to her:
1308: 20 marks for her expenses while staying at Rockingham Castle (JCD)
May 8, 1308: 10 marks for her expenses for her journey from Rockingham to the king (JCD)
1310: 100 marks for her expenses for her journey from Northampton to Berwick-on-Tweed to join Queen Isabella, plus another 20 marks as a gift from the king (JCD)
1313: Exchequer grants for her expenses: 10 pounds on October 10; 10 pounds on October 15; 5 pounds on Oct. 27; 5 marks on Oct. 29; 10 pounds on Nov. 7; 10 marks on November 19; 4 pounds and 1 mark on December 11 (JCD)
Feb. 14, 1314: 5 marks (JCD)
Easter 1316: Livery of green cloth, trimmed and lined with miniver. (Edward II, Isabella, Prince Edward, and the Countesses of Hereford, Warwick, and Cornwall, the latter being Piers Gaveston's widow (Eleanor's sister Margaret), also received the livery) (MV)
July 9, 1322: A life interest in the manors of Melton Mowbray and Sonyngdon, with a remainder to Eleanor's son Gilbert. (Calendar of Patent Rolls; the gift was made "out of affection to Gilbert")
September 1323: 13 pieces of sturgeon (AW) (Wisely, the King sent Queen Isabella 20 pieces, thereby avoiding the specter of sturgeon wars between the queen and Eleanor.)
1323: 100 pounds toward the expenses of Eleanor's illness while she was in childbed (JCD)
May 10, 1324, Life interest in the manor of Bramelhanger, again with a remainder to Gilbert (Calendar of Charter Rolls)
April 26, 1326: Grant to Eleanor of the goods of Alan de Newenham, whose goods had been taken into the king's hands after he was indicted for "divers felonies." (Calendar of Patent Rolls)
May 18, 1326: Grant to Eleanor of the keeping of the hundred of Gosecote, to hold until Stephen de Segrave's heir came of age. (Calendar of Patent Rolls) (On May 19, Edward issued an order to make the letters regarding this grant "as hastily as possible and deliver them to the bearer without delay.")
1326: 47 caged goldfinches (RH, MP, AW) (Why the odd number of 47? Were there originally 50, of which three died? Could only 47 goldfinches be found? Inquiring minds wonder.)
200 stockfish (JCD: author does not give the date)
Sugar bought to make sweets for Eleanor (MP)
100 marks on "one brief visit" near the end of Edward II's reign) (MP)
Haines also mentions "privy dining" between uncle and niece and his accommodating her at the royal manor of Sheen, and it's also been noted by Prestwich that medicines were bought for the two of them when they were ill in 1319-20. In 1310, a messenger was given twenty marks for bringing Edward news of Eleanor (JCD), the nature of which is sadly unspecified, and in December 1325, Edward offered prayers to the Virgin for his niece's safe delivery of a child, perhaps John le Despenser or Elizabeth le Despenser (Haines).
Why all of this generosity? There are generally three explanations: (1) Edward II was very fond of Eleanor; (2) Edward II was having sexual relations with Hugh le Despenser the younger, Eleanor's husband, and wanted to keep Eleanor sweet by showering her with gifts; (3) Edward II was having an affair with Eleanor herself. There's probably no way of knowing now which was the case. Both Michael Prestwich and Roy Haines have suggested the third possibility, based on allegations by a Hainault chronicler, Robert Reading's comment that Edward II was engaged in "illicit and sinful unions," and the lack of comparable entries in Edward II's records for other ladies. This can't be ruled out, but it's notable that Edward III treated Eleanor generously once he began to rule on his own and that he considered her to be a fit companion to accompany one of his sisters to her wedding. Though Edward III's court was not a prudish one, it still seems unlikely that he would have treated Eleanor so well had she and his father been slighting his mother by engaging in flagrant adultery.
As for the second possibility, though I think it likely that Edward II and Hugh were lovers, I doubt that the gifts were made just to keep Eleanor from raising a fuss about their relationship, whatever its nature. Had Eleanor been inclined to be petulant, Edward and Hugh probably would have simply stuck her on some remote manor and let her steam in solitude. Moreover, Edward II's generosity to Eleanor long predates his attachment to Hugh, though it's true that the large gifts date from the period when Hugh was the king's favorite.
All in all, I think that the first scenario--that Edward II was fond of Eleanor and treated her as a confidante--is the most likely one. It probably helped, of course, that the king was also fond of her husband.
James Conway Davies, The Baronial Opposition to Edward II
Roy Martin Haines, Edward II
Michael Prestwich, Plantagenet England
Malcolm Vale, The Princely Court
Alison Weir, Queen Isabella