While some students will gravitate towards geometric forms and other linear or sequential concepts, a ruler is usually helpful to all students at first. It simply provides a visual aid and an index to help conceptualize relationships. It doesn't even have to be a sequential or numbered ruler with quantities getting higher or lower like a gas gauge; it can simply be an incremental index as a visual aid.
Liquids can be conceptually helpful for students, especially in conjunction with a variety of containers. Containers can be many shapes -- in common geometric form from cylinders to boxes. Some of the containers can be indexed, such as a measuring cup. Differentials in relationships can be deduced when liquid is split between an indexed measuring cup and a nonindexed container. It can also be illuminating to use some indexed containers that have a continuous shape, such as a test tube, where one linear inch of additional liquid is equal to the previous inch, and some that indexed containers that taper so linear measures and volumetric measures differ.
Sand can be a useful manipulative as it behaves differently than water. For example, sand can be spread out on a table top and compared to a volume of sand in a container. This physical difference can illustrate concepts in ratios that is difficult with water, which must be contained more tidily or will dissipate.
Geometric shapes such as purpose-built teacher's aids or building blocks can help demonstrate increments in three dimensions. It can be advantageous to begin using same-shaped blocks to represent single quantities. The student doesn't need to jump ahead to more complex relationships and has a simple, tangible representation of basic fractions. Though shapes can be used to illustrated more advanced concepts too, such as the relationship of two equal volumes in varied shapes.
Money can be a helpful aid in that it is already assigned increments students are familiar with. With other manipulatives, the a whole unit is arbitrary. A brick, for example, can be one-quarter of four bricks or one-eighth of eight bricks, but a quarter of a dollar is always a quarter of a dollar.