Also have a dual purpose room. We did inexpensive white shelving units around two walls, with fabric boxes to stash stuff, (office, and my "crafty" supplies), threw in a few decorative items. Got a tall basket for rolled up mats/foam roller storage, a complementary lower basket to throw in the requisite elastic bands, small hand weights, etc. Decorated walls with pics from our travels, framed with colored mats.

Mirrors can look great outside in the garden to reflect greenery back and give an illusion of depth. If you have a courtyard it can help to create a look of more space and lushness of plants.

If it's a rental be a bit careful about adding holes to the wall. When hanging pictures of any kind don't go too high - around eye level usually works best. For easily moveable and inexpensive ideas think about: using a wide piece of fabric hung using a piece of dowl at each end to create wall art that you can change out quickly and easily. large seagrass matting to cover the floors. look for décor items that are will pick up the current theme and move with you - places like Tree of Life (if you're in Sydney) offer carved boxes for tables. Look at second hand carved teak blanket boxes (more storage and pretty) Think about batik style cushions in blues and greens to add a beachy feel.

How about a beautiful timber option? The below are Blackheart Sassafras but Black Butt, Spotted Gum.... Lots of choice and each one individual. Then choose some box frames and place randomly on the wall? DIY the interiors if you have the craft gene, with one colour included in each box to promote coordination. Make them personal or quirky or humorous and change them about as the mood takes.