The visits to the Coliseum and the Foro Romano are usually pretty tiring, especially in summer, because you will have walked so much. Our suggestion is to head back to the hotel to rest for a few hours. To return you can take the bus 63 from Piazza Venecia to Tritone-Fontana di Trevi (16 min and 1,5€). Once you exit the bus turn left to Via dei Due Macelli and you'll reach Piazza Spagna. If you still have energy you can walk there, it's about 20 minutes.
If you are feeling hugry, you can stop on the way at Ai Tre Scalini, at number 251 on the Via Panisperna, a pleasant Italian tasca in which you can try typical dishes and wine at reasonable prices. We suggest the Cacio al Coccio with Zucchine e Speck (house speciality), the Polpette al Sugo (meat balls), the Parmigiana di Melanzane and the Lasagna al Ragu. They serve it in small portions, ideal for trying several different dishes. As far as wine goes, we recommend a glass of Nero Davola (Sicilian red) or Gustraminer (white). Around 15€ per person. Monday to Sunday from 12.30 to 01.00.