Grab the contract missions that can be found on a town's message board and on the mast of the Adrestia whenever they are available. These are not story quests but rather tasks that you will complete anyway during gameplay so are basically free experience points. These tasks may be simple one-off assassinations or ongoing tasks that could take hours to complete.
Keep Advancing the Story:
Remember that it is not necessary to undertake every side quest and complete every optional objective that is presented to you. The easiest way to level up and acquire better equipment is to simply advance in the main stoy. Following the main and side quests will often take you into unexplored areas anyway.
Invest in Assassin skills:
As with Assassins Creed Origins this game offers three upgradable skill trees to customise your character (Hunter skills, Warrior skills, and Assassin skills). When you take into account that the best strategy to approach almost any enemy encampment is stealth where you pick off the enemies individually and engage the last few head-to-head it makes sense
That the Skills you need to invest in to employ this tactic are Assassins skills such as Critical Assassination, Stealth Master and Shadow Assassin.
If you've played previous Assassins Creed games you will know that Viewpoints show you all of the optional objectives in the surrounding area and also act as fast-travel points. Assassin's Creed Odyssey is one of the biggest titles in the series so make sure you save yourself a lot of time in the long run by synchronizing all the veiwpoints you see, even if you have to go out of your way to sync it.
When you start the game you will have a choice to play either Guided mode or Exloration mode. Guided mode is the Assassin's Creed maps as they have apperared in previous games while Exploration mode makes you ask around and get hints about an objective's location and forces you to find it on your own. You will find that playing Exploration mode will give you a much more satysfying experience as you will have to do the work yourself rather than rely on the game to do all the legwork.