The ATtiny85 chip is a simple $2 way to get into Arduino.
These off-the-shelf boards are brilliantly designed and great for one-offs. If, however, you need a bunch (or you're feeling crafty), you can make your own wearable board.
Stripboard (aka veroboard)
Chip - ATTiny85. Learn more about it here.
Chip socket - 8 pin
Dremel with cutting wheel (or a box cutter)
Soldering workstation (soldering iron, safety goggles, fume extractor, tip cleaner, etc.)
Programming shield for ATTiny85
Chip extractor tool
Clamp the board so you can cut it.
Using the dremel, cut the board to size: for an 8 pin chip, I cut a board 4 rows x 10 holes.
(Don't breathe the dust!)
Next, on the copper side, lightly cut a groove down the middle through the copper layer only so your 4 rows are now 2 columns.
Use a multimeter to make sure the left columns are completely separate from the right.
Note: You could do this with a boxer cutter instead of a dremel. Score the board by running the box cutter along a ruler clamped in place. Snap it along the score. Cutting this way is less accurate so make the board larger to compensate. The box cutter will also work on the copper traces.