If you have purchased Sony A68 with 18-135 mm lens, there is not many focal length you need to cover. However, prime lens always give better result and macro photography on APS-C body actually better if you have macro lens! Because APS-C crops, smaller size of camera, lighter lenses gives more flexibility. Choosing Sony A68 Lenses is Not Very Difficult Among A Mount Lenses. Sony’s Full Frame Cameras and APS-C Camera Are Compatible With Same Lenses. That 18-135mm Kit Lens probably better than many lenses sold by Sony. Unfortunately, optical quality of many lenses offered by Sony is not that good and if that 18-135 mm lens covers, it is probably need precaution. Of course Zeiss has some good lenses. We are not counting wild photography with A68 and mentioning few lenses which you may check detailed reviews on various websites. There is no meaning of purchasing cheaper lenses to expect degraded optical quality. Upgrading to A mount full frame means A99 at this moment. Others are E mount cameras. If you plan to buy E mount camera in future, you should buy adapter at present.
Sarcastically, your options are so less that choosing lenses become easy. You should read reviews of the lenses from individual lens review sites like
Which One To Pick For Future Sony A68 Lenses
APS-C has crop factor. Among prime lenses for APS-C A mount, Sony DT 35mm F/1.8 is actually around 50mm lens. Other two are one is a Macro lens with f/2.8 and the other is cheap China 30 mm prime lens (that costs half compared to DT 35mm). Sony made options just easy by manufacturing least options! More sarcastically, Sony DT 35mm F/1.8 actually overall similar to thousand dollar costly corresponding G lens. Of course, the body of the lens is just plastic and probably it is not safe to become curious to test it’s physical strength. It is slightly better in terms of bokeh effects compared to stock 18-135mm. The reason of considering this lens is it’s f. It is not wrong to think that 18-135mm kit lens is good, it is well built lens. Do not buy 50 mm f/1.8 to save more money – it’s actual focal length is 75mm.
Next is DT 30 mm F2.8 SAM Macro. Another cheap China lens. Good value for money but it has uncanny small diameter of lens elements. Honestly it is a rare collection. How Sony manufactures with point and shoot lens like diameter, that is not practical to guess. It does mean the lens need utmost care to prevent displacement of elements inside. Sony does have similar such for E mount but possibly this one has least diameter.
Next is DT 55–300 mm F4.5–5.6 SAM. It’s quality is exactly like stock 18-135mm.
All of the above supports full frame camera using APS-C size capture. I do not think it is prudent to invest to buy APS-C A mount lenses. Saving that money will help upgrade to a full frame body.
If your budget is just less, can safely buy DT 30 mm F2.8 SAM Macro. It will help you both macro and some normal photos. Unlike many forces ~50mm non-macro prime lens for APS-C, I will suggest DT 55–300 mm F4.5–5.6 SAM as next option. Sony DT 35mm F/1.8 is not bad for wider aperture but whether buying so many APS-C is good investment for your grand-children, I do have doubt. Many of us are actually using some inherited old full-frame lenses. It does not appear to me that in next 100 years, 35mm format going to change.
You definitely need a macro for creativity. At that price, it is really not much to expect. It is of course better than extension tube or magnification filters. Good magnification filters cost 50% of its price plus you can not get wider aperture with stock lens.
Next I kept Next is DT 55–300 mm F4.5–5.6 SAM for longer reach. You’ll agree that you need some glass to capture things you ca not see well like a bird flying. Build quality of this one really good. Unfortunately, as the starting f/4.5, you probably can not use it as macro at 300mm. So much light is not practical to supply for macro.
I kept Sony DT 35mm F/1.8 as normal lens. Sadly,except the wider aperture, I lack point to support. In other words, your kit 18-135 is not capable at so wide aperture, so you will find it useful if you frequently take photos at that focal length.