Do you have all the locations on the map and visited all them ? Many areas like the Brotherhood of Steel, the Glow or the Boneyard are optional. Walking around until the whole overland area is visible is a good idea, just be aware that you have a 13 years time limit.

The deathclaws in one of the Boneyard areas regenerate every few hours until you go underground and kill the mother.
You could also travel around in the NW corner of the overland map for random encounters, the supermutant patrols near the military base yield a lot of XP if you're able to take them.
Use first aid and doctor after combat to get even more XP.

A very cheesy way to get your XP up is pickpocketing, search an easy to pickpocket victim, maybe use some alcohol or drugs to lower its perception, stand directly behind it and keep on pickpocketing and planting the same small items over and over again.

Are you sure you have done all the quests? There are quite a few per each town and a lot of them don't become available unless you go back and revisit a town. If you really have done it all then wander around the map looking for fights, try the top left part of the map.

I've checked the Fallout wiki, and completed all possible quests listed there. I failed a couple of them, but I still completed the large majority of them. However, I haven't totally visited the northern region, so I'll try checking that out, and unfortunately I already killed the Deathclaw mother :(

http://www.the-spoiler.com/RPG/Interplay/fallout.8.html
-That's the website which you can use to know the best two options to level up. This guide helped me make it to level 21 quick and easy before going against the master. Since this is a walkthrough you'll have to scroll down to the "Gettin stronger still" section to know the ways of leveling up. This page also includes some tips and tricks to increase your character's "SPECIAL", get power armor and getting the best weapon for all the beginners out there.
-Hope it helps you and anybody out there trying to level up, just like it has for me. :)

Level 14 is pretty much the level you should achieve if you solve all the quests and visit all the places WITHOUT any grinding. See this thread https://www.gog.com/forum/fallout_series/fallout_1_what_am_i_missing]

The maximum level is 21. But it is impossible to achieve it unless you spend tons of time grinding. It's waste of time if you ask me. At level 14/15 you are plenty strong to annihilate all your enemies with little trouble.

But if you are really into it, then the best way to grind exp points is to either use the deathclaw trick described above, or simply travel around the mutants' military base and keep killing patrols.

hmm... i really think grinding to the XP cap would be a waste of time in the FO series.