I second the recommendation for a micro brand scooter. A bit more expensive than other scooters but very worth it.
Also, a balance bike. Try to skip tricycles and training wheels altogether, as it has been shown these are actually detrimental to learning how to ride a bike.
I would recommend against a trampoline, not for fun factors (of course they're fun), but because of safety issues. Studies have shown they significantly increase your chance of sustaining a spinal cord injury. Also, some insurance companies will also increase your premiums if you have a trampoline in the house/yard.
I hate plasticky things, but a water table has been wonderful. Mud or sand pits are also great. At our house, our kids get dirty when they play. We got over it pretty quickly as they get to dig, build, scoop, plant, and get to use their imagination.
Other goodies:
Simple sidewalk chalk or sidewalk paint.
Spray bottle. Fill it with water and let them "water" the plants and grass. Don't know if it's universal, but our kids LOVED them.
Some not so great toys (usually given as gifts):
Toy lawn mower: Everytime we were out in the yard, DD would ask us to bring it out. She'd push it out a little, and inevitably get bored with it. One of the adults had to go retrieve it everytime.
Cozy coope (and other toy cars): Very short window of use. Too little, and you're pushing them the whole time. Too big, and their knees don't fit in there anymore.
Kites: DS was given a kite for his 3 year old birthday. Kites are great, but for toddlers, just a source of frustration. Wait until they are older for kites.