Well, DG is actually my least favorite skill, even though it's one of my highest.
Aww
I think Agility has to be my least favourite, or maybe Hunter.. BLEH
Agility you can use serenity posts/bxp + lamps/troll invasion/jack of trades/barberry seeds on if you don't want to train it.
Hunter you can train with grenwall tickets effectively after you do Big Chinchompa minigame.
As for DG, well, you could try to get enough friends so you can get decent xp per hour, but if you don't you're pretty much stuck with soloing or finding random people, which is quite frustrating for me several times. You could try to do sinkholes, but you only get to do them a certain amount of times per day, but I'm that at experienced with them.
Hmm yeah those are some good ideas, the daily challenge for DG is pretty good for it, although you don't get many tokens so it may not be worth it for some :s