I'm pretty weak and I need to build some strength. What are some good arm strength building exercises? I can't go to the gym. Are push ups good? Is lifting a heavy backpack with your arms fully extended (to make it harder/heavier to lift) good?

How much strength do you lose if you don't train? I'd like to get to a good level of strength, and I would like to retain 80% of that strength without much training afterwards. Is that reasonable?

Thanks.