What Activities can I solo in WvW?

There may not be much you can do that will directly affect your realm's score, but you can certainly help put your team in a better position by disrupting enemy supply lines. Supply is absolutely vital with respect to upgrading your realm's towers and keeps, and denying the enemy team their own supply makes their larger structures easier to capture.

Most classes and builds should be able to flip un-upgraded supply camps without much trouble. You can also deny the enemy team by taking out their caravans. An unescorted Pack Dolyak is easy pickings for a solo player, and with the right build you might even be able to deal with upgraded caravans (which are escorted by two veteran guards). The challenge, of course, will be doing so before reinforcements arrive. Taking down a caravan will award you 3 points, but the real benefit is stopping the supply it carries from making it to a keep or tower.

The sentries located at crossroads and choke points can also be easily soloed. Capturing these points will grant you some WvW experience and give your team a single point. (It also removes a speed bump for a friendly zerg. Zerglings love standing in glowing rings.)