From site:

This activity begins with the reading aloud of several math-oriented stories, identifies key mathematical vocabulary terms, models math story writing, and then gives students an opportunity to write addition and subtraction stories. Students first draw a series of pictures which depict adding more or taking away objects; they then write a correlating story to go with the pictures they’ve drawn. Finally, students share their stories aloud, and equations are written which symbolize the adding and subtracting written into the stories.

This is a very-well detailed lesson plan with resources and storybook suggestions.  Some teachers have adapted it to their middle school classrooms.

From site:  On this website you will find thousands of our free mathematics enrichment materials (problems, articles and games) for teachers and learners from ages 5 to 19 years. All the resources are designed to develop subject knowledge, problem-solving and mathematical thinking skills. The website is updated with new material on the first day of every month.

LearningReviews.com note:  This site is hosted in the United Kingdom.  The problems are sorted by ages, with collections for ages 5-11 and 11-18 the grade levels are shown as “stages.”  Stage 1-2 problems would be for elementary age students in the US.

Each problems gives you a link for hints to solve it.  The solution link will not appear until the following month.

A+ Click is an interactive collection of more than 6000 math word problems for grades 1 to 12. Problems focus on common core standards for understanding, spatial reasoning and problem solving.

You can pick problems by grade level or by topic.  The topics range from basic arithmetic, algebra and geometry to logic, applied math and test prep.

The A+Click Test section has over 1,000 questions for each grade level, encompassing all the math skills for that grade.

Ad-free and requires no log in to use.

Note: Aunty Math has retired and is not longer available from DuPage Children’s Museum.  You can still view past challanges, though, using the Wayback Machine archive of this content.  Here is a link to Aunty Math 2008, with several past challenges.

Aunt Mathilda Mathews or “Aunty Math” (also known as Angela Giglio Andrews) periodically posted new math problems and logic challenges. These problems came from stories in her life and those of her two nephews Barney and Danny, and her niece Gina. The math challenges are designed for kids in grades K-5.

There are many different methods to solve Aunty Math’s challenges such as drawing pictures, searching for patterns, or use manipulatives such as buttons or beans.

From site:

The InterMath materials are organized into four key mathematical strands (Number Sense, Algebra, Geometry, and Data Analysis). Under these 4 strands, there are 13 units: Fraction and Decimals, Integers, Ratios Proportions & Percents, Quadrilaterals, Triangles, Polygons, Probability, Statistics, Solids, Circles, Graphs, Patterns, Functions & Equations. The InterMath problem set is comprised of open-ended investigations which are problems that have at least one of the following attributes: multiple solutions, multiple approaches to solving them, and multiple entry points.

As of this writing, solutions are not provided on the website.