This is the easiest way to get to Mtatsminda, which was the premiere park of Tbilisi. The funicular is cheap (2 GEL) and takes you up to a Soviet-era amusement park filled with games and children, though also with some high class restaurants.
The funicular is the easiest, fastest and funnest way to get to Mtatsminda. Though prone to shutdowns, being an easy uphill walk from the Parliament on Rustaveli, it's worth seeing if it's in operation. This will take you to the place with the best views of Tbilisi, where you can see all the way to Kazbegi mountain.
Don't bother with a taxi, just start walking uphill from the Parliament, the walk is easier than it looks.