Does anyone have any activities or lesson plans that can be used to teach students about hundreds, tens, and ones??? Any ideas would be greatly appreciated. My school does have the base ten blocks that can be used...I just need some ideas! Thanks.

Have your students make their own tens groups: stick 10 binder hole reinforcers on a strip of paper, 10 small stamped images on a circle or square of paper, whatever you have that is easy. Write 10 on each group. Make it be a 10s factory.
Then have them stack the 10s into hundreds and see how many the class produced.
Make a 10 frame: 2 rows of five squares as a work mat. Roll a dice and the kids put that many of the unit blocks on the 10s mat. (be sure they make 5 across before they go to the second row--reinforces the doubles nature of 10, two fives) When the frame is full exchange it for a long--they can match the units against the long to "prove" the long equals 10. If you have time, roll to match 10 longs to the flat (100).
You can do the same procedure with pennies, dimes and dollars. (I usually have them exchange for a nickel when one row if full, another nickel for the bottom row and then change both for a dime).
They can roll two different colored dice (or number cubes if you are not in Nevada). Put out that many longs/10s for one color and ones/1s for the other. Then you can exchange for 10s and hundreds more rapidly by adding the 10s and 1s from subsequent rolls. Students can put little cards with number amounts under the places and read them with each exchange.
I often have them work on a FLU mat: Flats, Longs, Units, just a 3 place work mat to keep the blocks in the proper spaces. I once got fancy and glued different colored construction paper to make three spaces glued to a backing. Laminated the whole mat and they lasted for years and years. I wrote Hundreds Tens Ones at the top of each space--it was helpful for some. It's a good project while watching a TV movie.
My big emphasis has always been "Whoops! There can never be 10 in the ones place. We have to move it over to the next place."
I figure you already do the 100, 10, 1 bit with straws or sticks or whatever to make the number of days of school.
It's very helpful for the students to see 10s are not just linear, but bundles, chunks, etc.

I have to use flats, longs, and ones in my lesson....I need an activity that will help teach hundred, tens and ones using flats, longs, and ones. Thanks.

If you have dice you can play the exchange game. Put the base 10 blocks in the middle (it helps if each has a place value mat 100's/10/1) and the roll 2 dice. They take the number of cubes the dice says. Everytime they get 10 cubes they exchange for a long. Play to 100. I stop the game every few minutes and make them write down the number they have on their mat for practice.

We also use the overhead alot - show a number of blocks, have them write down the number it represents.

Before I got base ten blocks I used drink straws. They are cheap and come in large volume. I asked parents to supply a package of 100 or more and then we used zip to bags with kids' names on them to hold their straws after we opened the original packages. My kids used rubber bands to bundle them into tens and larger rubber bands to bundle the hundreds. They did roll, but kids don't mind picking things up from the floor. In fact, when I had the younger brother or sister the next year, the parents often sent the same bag back the following year or just knew to get another package for the younger sib.