If you have nice neighbours you could ask them if you can use their internet by WLAN. (Although most providers forbid such practice. But they cannot control it.)
Otherwise, you could buy a USB stick UMTS or LTE modem (without SIM-lock), e.g. Huawei E180, E173, E303, or a UMTS or LTE Hotspot, e.g. Huawei E355, E5832. You find them second hand on ricardo.ch or ebay.ch, or new in some online shops.
And then use a prepaid SIM card with some kind of data option, e.g. Lebara
-> Saving options
Of course, since the amount of data ("volume") is limited, you must be careful not to use too much data, e.g. avoid video sites and music/video downloads!
Instead of using a specific modem/hotspot, you could use any smartphone that is capable of tethering, thus turning the phone into a hotspot. But if you just want to use ONE computer, a USB stick is IMHO the best solution.