Makeshift Weights
Who says you need dumbbells to get an arm workout? Try using a full water bottle (or two). Whenever your arms begin to feel tired, do a set of 15 curls (if you only have one water bottle, repeat for each arm). Take a 30 second break and do 2 more sets of 15. Once you're comfortable with this you can try other water bottle exercises - such as air-punches (literally repeatedly punch the open space directly in front of you with water bottle in hand).
[Photo via @maskulinmag]