There are some great suggestions on this thread! A new hobby, or volunteering for an organization that you're passionate about are both good.
I know that every 5 years, I had better do something to 'change up' my life a little. I love my husband, so I didn't want to change that, I generally change horses every 5 years or so ( learning to ride was a change I made after I graduated from college - that kept life interesting!) I've volunteered for a local wildlife project. But the surefire thing that always does it for me is taking a class. I need to learn something new to keep life interesting. I've taken classes in guitar, machine tooling, aeronautics, cooking and foreign languages. I usually need a 'kick start' every 5 years or so, but sometimes a little more often. Learning something new, and getting interested and passionate about something outside my normal life keeps me interested in life, and helps me to look at things in a new way.
(Of course, I love to travel and I always look forward to see what everyone is up to on the DIS, but don't those go without saying on this board??)
There are so many options out there, it's easy to get overwhelmed by the possibilities. Just jump in to something new once in a while!
Let me also say .. some of us feel 'in a rut' when we're really just tired and worn out. Take a little time for yourself, eat well, have your nails done, get a massage, take up a sport, stretch or go dancing. Stay healthy and life will seem much more inviting and sooo much easier.