I guess everyone's idea of fun is different, but my fun bike is my YT Jeffsy (full-suspension 29er trail bike). I bought it last year to replace a 10 year old Gary Fisher hardtail, and I wasn't sure how much I'd actually end up riding it considering I hadn't ridden the Gary Fisher much in the previous years. Turns out it's an absolute blast to ride and has completely reignited my passion for mountain biking, to the point where I'd rather do that over road riding most of the time. I probably average one or two real rides a week, but I also use it to cruise around the neighborhood with my daughter when she rides her bike.
As far as maintenance I guess it's on the fussier side as most FS mountain bikes are. Hydraulic discs, two shocks and a dropper post means more things to maintain but they are also built to take abuse. Drivetrain parts tend to wear out kinda quickly because of the dirt and grime and constant shifting. Totally worth it considering how fun it is to ride though.
I will say it's probably not the best option for the use case you described. If I were going to buy one bike that I could use on urban bike paths, around town, and off-road, I'd probably go with a fun hardtail mountain bike like the
Commencal Meta HT or
Santa Cruz Chameleon with flat pedals. Both can run 27.5"+ or 29" wheels which gives you lots of options for different terrain/handling.
ETA: To your question about transport, the main reason I don't ride my mountain bike more often is because I have to put the rack on my car and drive to all the good local spots. If I could ride to them I'd do it much more.