Step 1: Assemble the Items You Need to Carry With You
This may include the usual suspects: phone, ID, keys.
Step 2: Consider the Angles.
If you need a flat space, try the outer side.
Step 3: Consider Sweatiness.
Keeping things on top will make them less vulnerable to heat than keeping them on the side or underneath. This may or may not be an issue, depending on the item. You won't want electronics to get damp, and you won't want lipstick to melt.
Step 4: PROFIT.
Leave the house with everything you need and no bag to lug around or worry about.