If you have HNO3, dissolve one of these nickel tablets and make a solution of nickel nitrate (with still quite some residual HNO3 left). Nickel salts have a fairly extensive aqueous chemistry, with lots of complexes and nice colors.
Beware though, dissolving nickel in HNO3 produces very toxic NO/NO2 and nickel salts are said to be carcinogens, so be careful.