What Should I Do Next?

Get 83 woodcut and then you can easily get 99 firemaking for free at HIGH xp rates. Jadinko Lair burning Curly roots.

 

With double XP coming up, i figured i'd get back into the game. What should i train to take advantage of the bonus?

 

I've compiled this for you, hope it helps.

Runecrafting - Go to runespan until 99. Easy fast xp.
Agility - Ape atoll course until level 80, gnome advanced until 85 with summer pies (should get agile), and then barbarian advanced with summer pies until no fail level until 99.
Thieving - Pyramid plunder is going to be your best xp below level 80, but if you've done do no evil go to monkey knofe fighters at 80+, otherwise stay at plunder. Sorceress garden is too laggy.
Crafting - Battlestaves if you're rich, or green dragon hide bodies if you're slightly poorer. Neither method is cheap, unless you want to craft 400k gold amulets.
Fletch - Do yew longbows until 85, and then proceed to do magic longbows until 99. Easy xp. Easy skill.
Hunter - Falconry is recommended until 70, then I'd do red sallys until around 85, as then you can do grenwalls with a decent amount of xp and cash.
Mining - Granite/LFM until level 80. You make no money, but it's fast. 80-99, do concentrated gold deposits at LRC. They are good cash, and good xp also!
Smith - Mithril Platebodies until 82. Adamant bolts until 88. Adamant Platebodies until 99. Fast quick xp.
Fishing - Monkfish (do swan song, trust me) until level 85, cavefish 85-90, and then rocktails until 99. Mad cash.
Cooking (with gauntlets) - Lobsters until level 81, swordies until 90, monkfish until 94, and then rocktails until 99. If you're lucky, the rocktails sometimes profit.
Woodcutting - 66-75 do willows or maples. Just get 75. Once you've got that, ivy is decent xp/hr (68-75 isn't worth doing there), so then move to ivy and get easy xp.
Farming - fruit trees and normal trees if you are rich. If you aren't, herb runs until you can afford the trees.
Lastly, firemaking - Jadinko Lair, it's free xp, and it's like 250-300k/hr, so eat your heart out, get 99 in no time.

Hope I helped

 

It can't hurt to cannon flesh crawlers for effigies for the time being. Even though it's expensive, you would be able to put the experience to use on any sort of skill. Also, I would suggest knocking Prayer out of the way, and pursuing this goal even though you only have Attack and Strength to max. Personally, I'll being using the cannoning method for 99 Slayer since RSGP is remarkably cheap.