Other than Bag of Holding. If possible, i'd like to spend all of the 2500 instead of using 2000 or 1000, but those are welcome too.
If you have Improved Grapple then take Armbands of the brawler for only 500g adds +1 to grapple/grapple defence.
Unless you use a monk weapon then just save the cash for that first belt of +stats or the Amulet of mighty fists etc.
Had a grapple based monk and it's pretty funny, well it was until they house ruled I couldn't grapple and tie up a Gelatinous Cube any more...
If you already have the basic stat-boosters, consider buying a cloak of the hedge wizard (transmutation) for 2500 gp - enlarge person and expeditious retreat can both be quite useful for a melee combatant, and at-will mage hand and prestidigitation never hurts. Divination could also be nice to help the party with detection spells and pull off essential combat maneuvers.
Alternatively, you could use the money to purchase several less expensive items for ultimate versatility. That much money could buy you a cracked dusty rose prism ioun stone, an ioun torch, sleeves of many garments, a traveler's any-tool, and a tree feather token - all with 1000 gp left over to get bracers or a cloak.