But they're big. I have had a go and can report that it is possible to eat a whole one in one sitting, but I am known for my extreme capacity in that department. So, in the interests of those of you without Man vs. Food style eating abilities here are my top tips for things to do with a watermelon.
You must, at some point in your life, do the last one. Maybe only just once. But it needs to be done. Trust me on that.

I take no credit for this idea (though I'd like to!), but just look at this little beauty. The 'base' is a slice of watermelon. Clever, huh? This is a fun idea for kids (and adults for that matter!) at picnics - a proper showstopper.
This comes from this site which claims to tell you 'everything you want to know about watermelon'.
You may recall my recipe for f*ck me oranges? This also works a treat with watermelon - just slice thinly and arrange on a plate, drizzle with rosewater and a little cardamom. Gorgeous eaten sat out in the sun, with a little chilled prosecco perhaps?
Slush
Chop a couple of handfuls of watermelon. Put into freezer bags and pop in the freezer.
When needed, just whizz up a glass full and dilute a bit with water to the correct consistency.
Not only is it damn tasty, it's pink. Enuff said. Also good if you add a few strawberries to pimp up the pink...
Mr Strongman Fitness picked all the pips out of his (sigh) so this is worth considering if you don't like the seeds. Apparently this is to avoid a 'watermelon tree growing in [his] tummy', just so you know....
Icecubes
Same principle as above, chop into cubes, freeze and use as ice cubes. Defrosted watermelon that's been soaked in gin and tonic is very tasty, just so you know.

Watermelon works very well in salad with something a bit salty. I happened to have leftover feta, but halloumi is also very good.
For the vegans, just add a few more olives and capers and job done.
Quantities are for one hungry person, so adapt as appropriate!
250g asparagus, trimmed.
25g feta, cubed.
50g watermelon flesh, cubed.
1 tblsp black olives.
1 tsp capers.
Juice of a lime.
Drizzle of oil and coriander leaves to serve.
Quickly blanch the asparagus for a couple of minutes, drain and leave to cool slightly (nice when still just warm). Mix with the other ingredients, toss together and serve scattered with the coriander and drizzled with oil (I use bergamot oil here from local company Positively Good For You, which is particularly good).
Tastes of summer :)
Gazpacho
Cold soup is something you either love or, well, is a bit meh. I think this is fab, Mr Strongman Fitness was more down the 'meh' route. Try it and see what you think...
2 tomatoes
Half a cucumber
Half a red onion
200g watermelon (about four handfuls chopped if you can't be bother to measure it).
Half a chilli
1 tsp red wine vinegar
Stick it all in a blender, whizz.
Great garnished with a couple of the watermelon ice cubes from above and a handful of chopped herbs (mint, basil etc are good), or a bit of leftover feta or similar. For a posh 'do' I'd also consider adding some tequila and a bit more chilli I think, and serve in margarita glasses but that's just me being a bit pretentious.
The watermelon of doom
1 whole watermelon.
1 bottle of clear alcohol of some form (I used vodka, but malibu or similar would be good).
Start about 24 hrs before needed. Use an apple corer to take out holes of skin just in the very top of the melon. About four works well. Start trickling the alcohol into the holes. You'll need to do this slowly and often.
When enough alcohol has seeped into the melon (I'll let you decide how much is enough), slice the top off, and give everyone a spoon.
Lethal. Please drink responsibly.... :) :)