Just a suggestion if you want to add an interesting bit of side mission challenge to the campaign. I was playing as Trevor and did the side mission with the crazy mountain bike chick Mary Ann. Afterward I rode the bike to the next side mission I saw, Josh the real estate guy. I took a direct route, riding over mountains and back roads.
Then I decided to stay on the bike to do Josh's mission. Most of the houses he sends you to are in the mountainous area of Vinewood Hills, so if you try to take as direct a route as possible from one house to the next, you can get in some good minor mountain and urban trials riding, bunny hopping the bike here and there to get over obstacles.
At times I was pretty surprised at what you can do with the mt bike, and there are some stunt jumps here and there to add to the thrills. I also made a few jumps of my own just from natural features, like dirt slopes near pool decks. Sometimes it's amusing to see the reactions of the rich people freak out as you come flying into their backyard pool area.
In case some of you didn't know, you can bunny hop the mt bike by hitting space, then adjust fore and aft lean with L Shift or L Ctrl. Trevor can pretty much sprint non stop on the mt bike once you build his stamina a bit. Ironically it seems you have to prep Franklin for it more than he. Last time I tried Risk Assessment as Franklin he kept getting exhausted.