I would start with a new shower curtain. Here is a suggestion:

http://deciture.com/shop/bath/9/venice-shower-curtain/
Hope this helps :)

i have a very similar bathroom that i'm hoping to redo as well! Our ideas have been to stain the cabinet (and medicine cabinet) a darker mocha color, re-paint the walls (grey, sage green, or a dark tan/beige), new light fixture, and new faucet. in your case, I would also change the shower curtain to a color or slight pattern that would complement the rest of the room. You could even hang it higher (give the room the illusion of more height), or use one of those curved shower curtain rods for additional space inside the shower!

How do you change the tub to just a shower?

Rip the tub out and put a shower in, if you are going to do that you might as well redo the whole bathroom.

Tub needs replacing anyway. Had been patched already,, before we purchased the home 10 yrs. ago. Am planning to paint and make small changes.

I would swap out the shower head and faucet with this piece. Easy install for a handyman familiar with changing a faucet. We just re did our bathroom and it was affordable less than $300 delivered. Considering a massager and facuet is over $200. Great deal and nice upgrade.

http://www.amazon.com/AKDY-Aluminum-Az51371-Massage-System/dp/B002ZK4PAE/ref=pd_sim_sbs_hi_56