I've finished the main questline and all the guilds quests..What are some things I could do to spice up the game a little

Roleplay, explore, do other small quests in Towns ( I know for a fact you didn't finish all side quests ).

There is plenty to do.

Side quests are a bit dull to say the least

Clear every dungeon on the map with no fast travel.

It's something to do, isn't it?

don't use fast travel and don't power level skills like enchanting and smithing

Throw all your weapons and armor away, then start again at your level finding equipment.

Dugneon crawl. Go hunting. Make a save and kill an entire town. You know, the usual. Some of the side quests aren't that bad, and a few of them get you some really great stuff. Also searching for all of the words of power poses a challenge and generally offers some pretty great equipment. If anything elase, you can get some stuff to disenchant. I found an axe of fiery soul trap, disenchanted it, put it on a sword and gave it to my companion. No she slaughters stuff and traps sould for me. It was great.

*edit* Speaking of killing an entire town, I did this in Oblivion, so much fun. I had like a 100,000 gold bounty.

Find all the Dragon Priest masks and display them in Labyrinthian.

Complete all the daedric quests and collect all of the artifacts.

Buy and upgrade every house.

New character.

Make a new character with a different playstyle. Don't grind crafting. Don't use alchemy in conjunction with any other crafting skills. You will enjoy the game more.

Daedric Quests, if you haven't done them by now. They are among the best quests in the game.

Roleplaying
New character with a race you haven't tried before, and a new playstyle.

I keep a personal rule that all my characters can join only one faction + Legion/Stormcloaks, and must focus on one combat type.

Me too. I can't understand people who do all the factions with one character. Would the Archmage really go on a killing spree?

These are some good ideas. Thanks guys