Find some medium sized books (or unopened wine bottles like me) and hold them palms facing forward. Keep your elbows glued to your sides and slowly (5 seconds) squeeze you biceps and curl the bottles up toward your shoulders. Then slowly (5 seconds) lower back down to starting position. The slower you make the tempo, and the harder you squeeze to engage your bicep, the more difficult you can make this exercise. That's always one way to increase difficulty when you have limited weight options.