Senate seats are elected state-wide, so they largely go the same way as the presidential vote. If you're in a deep-red or deep-blue state (i.e., nearly all of them), your individual vote isn't going to make a difference.
House seats are district-specific, but:
a) the re-election rate of incumbents is over 90%
b) districts are often drawn to lock-in control for a specific party
State senate and house seats are often no better.However, much to the credit of the sibling response, there are all kinds of local and regional races as well as ballot initiatives that are important.