Straws

I'm a big fan of using semi-occluded vocal tract exercises for warming up. As well as lip trills and bubbling, I have been using straws in the rehearsal room and in the recording studio. Making kazoo-like sounds through a straw allows you to warm up in a way that causes less stress on the vocal folds as it helps us to use the optimum amount of air-flow. Using a straw also has the added benefit of making the vocal tract longer, optimizing it to reinforce the acoustics and aerodynamics of the vocal fold vibration and essentially training the vocal tract to do the same thing without the straw.

Tom Burke has an excellent video called "Straws Will Save Your Life" that explains how to exercise with straws, that I recommend you watch.