Pair up light and dark 3 1/2 blocks right sides together. Draw a diagonal line on the wrong side of the light. Stitch a 1/4 on both sides of your drawn line then cut on drawn line. You will end up with two HST blocks. You can trim one of them down to be 2 1/2 inches to mix and match with your 2 1/2 squares and you can square the other one up to 3" HST. There are countless things that can be done with a mix of squares and HST blocks. If the quiltville site doesn't give you enough ideas you can invest in a book specifically for scrap quilts or see if you library has one.
Edited to add, I have made the depression block. It looks great when done and is great fun to play with HST. another one along the same lines is Aunt Sukeys choice, simply replace the flying geese unit with two HST. Looks great done scrappy with matching values.
http://www.quilterscache.com/A/AuntS...oiceBlock.html