Archaeological Dig
Use a sandbox or find a great spot at the beach to bury plastic dinosaurs and/or bones in the sand. If you do this at the beach, mark out the location with some tent stakes and string. Also, remember how many items you hid. Assign points to different types if you want to give the game an extra twist, but it's not necessary. Kids will enjoy the thrill of digging for the buried fossils regardless. If possible, give each child a pail and a shovel. Then, let the hunt begin. The child who finds the most dinosaurs/bones in the allotted time wins. If the children are very young, let them keep the dinosaurs they find as a prize.

Panning for Gold
Spray paint some tiny stones gold and use a hard plastic swimming pool or sandbox to mix more unpainted stones and lots of sand and water. For this hunt either get a steel sieve for each of the kids (they are available at most discount stores) or cut pieces of plastic screening for them to use to pan for gold. The child who finds the most gold in the allotted time wins. Please note: It is important that the stones you use are very tiny or it will be too easy for the children to spot them without sifting through the sand and other stones.

Jigsaw Puzzle Hunt
Get a kid's jigsaw puzzle that has large pieces (20 or less is optimal). Hide the puzzle pieces all over the play area. Kids have to find the pieces and put the puzzle together to win. If you want more than one team to compete, paint the backs of each team's puzzle pieces a different color.

Clean Up Hunt
Make a list of things that are routinely left around the yard and have your children compete for who can find the most items in the allotted time and put them back in their proper place.

Nature Scavenger Hunt Bingo
Make up some blank bingo cards and get the kids to write or draw in a different nature item in each square. Then, have them trade cards with someone else and send them out to find the items on their card. Play till someone gets a bingo or a full card.

Internet Scavenger Hunt
Rainy days are perfect for an internet scavenger hunt. There are usually several available if you do a search on the web. Some even offer great prizes. Bonus: You'll have fun visiting interesting sites together with your children and you'll probably learn a few things along the way."