We have been there, but the only hiking we did was from the RV campsites to the crater and back.
Be warned that there isn't a lot of shade in the crater. This time of year, you better be *really* prepared. Take plastic buckets, shovels and possibly a pickaxe or mattock, and a screening box (you might be able to buy a screening box there, but call and check first). They provide a place w/water to do your screening.
The campsites are pretty decent there. Actually, they are pretty decent in many AR state parks.
Other AR state parks that are good for hiking is Devil's Den and Petit Jean. Neither are what I would call close to Crater of Diamonds, but you might consider them too if time allows.
There is - or at least was - a decent Mexican restaurant around the downtown area of Murfreesboro, in case you get hungry.
C