Standard for Mr. Beers with booster is 2.3% for the malt extract can, and 1.4% in the booster. Assuming you don't use anything else in the mix, you end up with 3.7% ABV
Many of the recipes could have a bit of honey or brown sugar added, but I wouldn't exceed 1cup per standard kit, or it's going to throw off your flavors a bit.
Now, if you start with the premium kits that use either 2x Hopped Malt Extract, or 1x Hopped and 1x Unhopped (4.6% ABV), you can toy with that a bit further, but with the unhopped keep the style in mind- when you add more sugar or honey, not all of that is going to attenuate, which will mean some added ABV, but also a bit more sweetness at the end. If you don't add more hops, you might throw off your balance a bit, and actually make a less tasty brew.
You could also just pick up some dry or liquid malt extract to add to your brew, because the yeast attenuates those better, you can mess with the recipe a bit further before you really notice the profile, and particularly the hop bitterness and aroma, are effected.
Remember, low alcohol doesn't mean poor flavor, and high alcohol doesn't mean tasty.