After killing a Perverted Polypede your weapon is upgraded to the next available, unless you already have the iSword. Otherwise, your armor is upgraded. If you already have the best armory money can buy, you'll receive Mega Poly Armor!
Don't buy Intelligence Potions or anything like that if you're already at 25 since it won't work. However, you can go over 25 in a stat if you upgrade yourself to 25 and then gain levels. Note: I can't honestly tell you whether or not it makes a difference though.
Many rooms in caves have random gold on the floor. The second room of the Giant Zombie Bunny cave, for example, gives gold after you search anywhere a couple times.
Searching logs and pools of water can give you HP back, but they do wear out. Give them a rest and try again later.