Most items that duplicate class abilities are daily use and seem to add a +1 level modifier if it's an at-will, +2 if it's an encounter (and I can't think of any daily powers that are added as daily item abilities - that would be pretty potent). Most crit abilities seem to be a +1 modifier, damage type modifiers (ie: as a minor action activate the ability, as a minor de-activate it, while active all attacks gain the fire keyword) seem to be +1.
If you can't find a class ability similar enough to what you want to work I would suggest you first approach the problem by thinking "If this were an ability, what type would it be?" Then you can apply the above guidelines as needed. Also remember that combining certain abilities together can result in overly powerful combinations.

oh wow! awesome ideas! i have been going over the dmg and phb trying to find some powers that made sense but i wasnt really coming up with much. i really like the lashing out thing and the "caring for" thing. although i dont want the new item they find to be a burden to them and make them annoyed. slightly annoyed is okay, but i dont want them pissed off.

i like the healing potion thing, but im not sure how i would do the whole "have to be sunlight" thing...

Perhaps just water. It's vines could possibly act as a locked gauntlet once it's wielded. So as long as its looked after it wants to be with its master.

What he said...

But only give him the Healing Fruit if he talks to the vine.

"Ohh.. you're such a good vine."
"Time to go to battle."

or maybe i give it a +2 bonus to AC when in forest/wooded areas. i'm afraid the "taking care of it" part my make the item a little too comical. not that im against the funny stuff, but i want this item to be a "serious" deal. although, having a living shield would probably make for better memories down the road....hmmm...

what if i made them feed it with some of the growth chemical they have, but with the potential consequence that one day it could take over and start to attack them? see this is the reason i want better guidelines on how to create magical items...im too worried about throwing the balance of the player powers out of whack. i dont want my epic monster down the road to be easily defended ya know? i guess the monster taking over the shield/player would be my escape clause/nerf.