Here are some plants they CAN eat:
Alfalfa, basil, borage, checkweed, coltsfoot, dandelion, goldenrod, green clover, groundsel, mallow, plantain, yarrow, asters, marigolds, nasturtiums, sunflowers and sweet peas.
These are some plants that are toxic and they CANNOT eat:
Buttercups, deadly nightshade, scarlet pimpernel, flowers and leaves from bulbs such as tulips, Lily of the valley, rhododenrens.
Safe Tree Wood:
Apple (not seeds), arbutus, aspen, apricot (must be dried for at least a month), bamboo cane, crabapple, maple, mesquite, mulberry, pecan, peach (dried for at least a month), pear (not seeds), rose (all parts above ground).
Here is a list of some toxic tree woods:
Boxwood, cashew, alder, almond, beech, balsam, fir, birch, cedar, cherry, all citrus woods, cypress, ebony, elm, eucalyptus, fig, fir, hemlock, horse chestnut, juniper, laurel, magnolia, mahogony, nectarine, myrtle, oak, oleander, olive, pine and pine cones, plum, plywood, spruce, teak, walnut, sequoia, yew.
These are not complete lists, so please check at the ASPCA site if you are in doubt about any material.
Your pet shop carries wood and wood toys that should be safe for your rabbit or guinea pig to chew on. But please make sure of the types of wood used and that the wood is not treated and is all natural.
If you suspect your pet has ingested toxic material, contact your vet immediately.