26" wheel and Continental make a Very thin tyre called a GP. It is about the width of a road bike tyre and takes high pressure-And is a folding lightweight tyre. So any narrow tyre that is lightweight and takes High pressure of around 100psi is what you want.

Without changing major components there is not a great deal to be done to change a bike like this, and it does sound as though you will be investing in a new Lightweight bike in the "Near" future. One exception though is wheels. Find a Wheel builder and get a wheel built up like An XT Hub with Mavic 717 rims. OR look around for some Mavic Crossrides on sale at an online Store. The wheels may not be a great deal lighter than your existing ones- but they will be a darn site better built and will make a difference in performance.

The "Wastefull" things to change, unless you are keeping the bike, are The handlebars- Seat and seat post and the bar stem. These will cost a lot to get in a lightweight form so it might be better to change the bike.